experiment 731

The horrifying crimes and brutal experiments of the secretive WWII Japanese ‘Unit 731’

During World War II, deep in the occupied lands of Manchukuo, Japan operated a covert program known as Unit 731. A secretive compound became a horrifying epicenter for experiments on thousands of men, women, and children from China, Korea, and other countries.

They even kept meticulous records which cataloged their subjects’ suffering as if they were merely data points. 

How did Unit 731 begin?

In the early 1930s, as Japan expanded its imperial ambitions across Asia, the establishment of Unit 731 was a very dark chapter in the country’s military strategy.

It was created in 1936 under the leadership of General Shiro Ishii, in the occupied territory of Manchukuo, in northeast China.

The region, which was strategically valuable for Japan’s empire, provided an ideal location for the Japanese military to conduct its secretive biological and chemical warfare experiments.

The Japanese-controlled city of Harbin became the operational hub for these activities, and the sprawling complex of Unit 731 was constructed nearby in the Pingfang district. 

Shiro Ishii was a microbiologist and army officer, and he proposed the development of a specialized research facility to enhance Japan’s capabilities in biological warfare.

He argued that diseases could become powerful weapons, capable of crippling entire populations without deploying conventional military forces.

By 1936, Ishii’s vision materialized as Unit 731. 

From the beginning, the Japanese government provided full support for the project.

Funding and resources flowed to Unit 731, which operated under the guise of the Epidemic Prevention and Water Purification Department.

This cover allowed the facility to conceal its true purpose, even from many within the military.

The remote location in occupied China enabled Ishii and his team to conduct experiments without interference from the international community.

Local civilians and prisoners of war , including Chinese, Koreans, Russians, and other nationals, were forcibly taken to Pingfang and were the test subjects for these experiments. 

Who were the masterminds behind Unit 731?

Under Ishii’s direction, the unit developed a hierarchical structure to ensure efficiency and secrecy.

The Pingfang facility housed several divisions, each responsible for specific research and operations.

The First Division focused on human experimentation, testing the effects of diseases like plague and anthrax on prisoners.

The Second Division concentrated on the development of biological weapons, including bombs designed to disperse infected fleas.

Supporting these efforts, other divisions managed administrative tasks, logistical support, and the production of experimental materials. 

Among Ishii’s key collaborators, Lieutenant General Masaji Kitano, who was also a physician, oversaw many of the facility’s day-to-day operations and succeeded Ishii as the head of Unit 731 in 1942.

Other notable figures included Colonel Tomosada Yoshio and Major General Kawashima Kiyoshi, who were instrumental in managing field tests and implementing large-scale experiments.

These leaders worked closely with a network of Japanese scientists and medical professionals, many of whom were drawn from prestigious universities and research institutions. 

Personnel who worked within the unit were sworn to secrecy and faced severe consequences for any breach.

This oppressive environment, combined with Ishii’s manipulative leadership, ensured that the atrocities committed within the walls of Pingfang remained hidden for much of the war. 

What were they trying to achieve?

In its inception, Unit 731 had the singular and grim goal of advancing Japan’s capabilities in biological and chemical warfare.

At its core, the unit sought to develop weapons capable of devastating enemy populations by spreading infectious diseases like plague, cholera, and anthrax.

These pathogens were chosen for their ability to cause widespread illness and death, which meant that they could incapacitate entire regions without the need for conventional military engagement. 

Additionally, Unit 731 prioritized large-scale experimentation to understand how diseases affected the human body.

Prisoners were infected with various pathogens through injection, ingestion, or exposure to contaminated materials.

The purpose of these experiments was to study the progression of diseases under controlled conditions, which was essential for weaponizing these pathogens.

Researchers aimed to refine delivery mechanisms, which included biological bombs filled with infected fleas and aerosols capable of dispersing pathogens over large areas. 

Meanwhile, experiments were conducted using substances like mustard gas and cyanide, which were tested on live subjects to observe their effects on human physiology.

These tests were designed to determine the most effective concentrations and methods of deployment.

The unit also investigated methods of combining biological and chemical agents to maximize their destructive potential, which was an unprecedented approach at the time. 

Shockingly, Unit 731 also focused on creating methods for their deployment on the battlefield.

The unit’s field tests in occupied China often involved releasing pathogens into unsuspecting villages to assess their effectiveness in causing outbreaks.

Such operations were integral to the broader strategy of perfecting weapons that could be deployed quickly and with devastating consequences. 

Experiments and atrocities

Vivisection was a particularly gruesome practice at the facility, conducted on live prisoners without the use of anesthetics.

Scientists aimed to study internal organs and bodily functions during the progression of disease, which meant that subjects were often dissected while still alive to observe the immediate effects.

These procedures involved cutting open abdominal cavities, removing organs, and analyzing the damage caused by infections or chemical agents.

These experiments also included women and children, demonstrating the utterly indiscriminate nature of these atrocities. 

Meanwhile, frostbite experiments subjected prisoners to extreme cold to study the effects of hypothermia and frostbite on the human body.

Prisoners were forced to endure subzero temperatures, often by plunging their limbs into ice water or exposing them to freezing winds.

Once frostbite set in, researchers struck the affected areas to determine the extent of tissue damage.

Specifically, they used instruments to measure the breaking point of frozen flesh.

These experiments were intended to help the Japanese military develop effective treatments for soldiers operating in cold environments.

However, the suffering inflicted on the subjects often resulted in severe injury or death. 

Furthermore, Unit 731 conducted weapons testing on living prisoners to assess the lethality of various biological and chemical agents.

Bombs containing anthrax or plague-infected fleas were detonated near groups of captives, who were intentionally exposed to gauge the effectiveness of the dispersal mechanisms.

In other instances, prisoners were exposed to poisonous gases like phosgene or mustard gas to measure their effects on respiratory and skin systems.

Researchers meticulously recorded the results, including the time it took for victims to succumb to their injuries. 

Who were the victims?

Among the victims of Unit 731, the majority were Chinese civilians and prisoners of war, who were forcibly taken from their homes or captured during military campaigns.

It is estimated that between 3,000 and 12,000 people were subjected to experiments within the unit’s facility in Pingfang alone. 

Additionally, Korean prisoners, who were often seen as expendable due to Japan’s colonial control over Korea, were frequently included among the subjects.

Russian prisoners of war also suffered within the facility, particularly during the later stages of its operation when the Soviet Union became a primary adversary.

Some accounts suggest that Southeast Asians and Allied prisoners of war, such as captured Americans and British soldiers, were also used in experiments, though these cases appear to have been less common compared to the mass use of Chinese and Korean captives. 

The exact number of victims remains difficult to determine due to the secrecy surrounding Unit 731’s operations and the deliberate destruction of records at the end of the war.

In fact, some estimates place the total death toll, including those killed in experiments and field tests, at over 200,000 when considering the broader impact of the unit’s biological warfare activities.

This figure includes the indirect victims of disease outbreaks caused by weapons developed at the facility, which were deployed across occupied territories with devastating consequences. 

Post-war cover-ups and immunity deals

After the war, the Japanese government and military leadership worked diligently to conceal the activities of Unit 731.

Many documents detailing the unit’s experiments were deliberately destroyed, which was intended to erase evidence of the atrocities committed.

General Shiro Ishii, who was the unit’s head, instructed his subordinates to burn records and dismantle the facility at Pingfang before the arrival of Allied forces.

As a result, much of the evidence that could have been used in prosecutions was eliminated, leaving investigators with only fragments of the truth. 

In the immediate aftermath of Japan’s surrender, Allied forces, including the United States, began investigations into Japanese war crimes.

However, the pursuit of justice for Unit 731’s victims was hampered by geopolitical considerations.

The United States, motivated by Cold War tensions and a desire to acquire scientific knowledge from Unit 731’s research, negotiated immunity deals with many of the individuals responsible.

General Douglas MacArthur , who was overseeing the occupation of Japan, approved these agreements.

In exchange for providing data on biological warfare experiments, figures like Ishii and other key members of Unit 731 were spared prosecution.

This arrangement allowed them to escape trial at the Tokyo War Crimes Tribunal. 

Meanwhile, the Soviet Union conducted its own trials for some members of Unit 731, including several lower-ranking officers captured in Manchuria.

These proceedings, held in Khabarovsk in 1949, revealed details about the experiments conducted on prisoners and the use of biological weapons.

However, the United States dismissed the findings as propaganda, which undermined broader efforts to hold the unit’s leaders accountable.

The immunity deals and lack of international consensus on prosecuting Unit 731 further ensured that the full scope of the atrocities remained hidden from the public for years. 

Following these developments, many former members of Unit 731 reintegrated into Japanese society, securing influential roles in academia, medicine, and business.

Ishii himself lived a quiet life after the war, avoiding public scrutiny due to the protection afforded by the immunity agreement.

Other individuals, such as Masaji Kitano, who was Ishii’s deputy, became prominent figures in post-war Japan’s medical community.

This failure to prosecute allowed perpetrators to avoid responsibility and contributed to the broader suppression of information about Unit 731’s crimes. 

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Inside Unit 731, Japan's Gruesome WWII Human Experiment Program

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Key Takeaways

• Unit 731, a Japanese Imperial Army program, conducted deadly medical experiments and biological weapons testing on Chinese civilians during WWII.
• Thousands of prisoners were killed in cruel experiments, and perhaps hundreds of thousands more died from biological weapons testing.
• The true extent of Unit 731's actions was shielded from public knowledge for years, with the U.S. granting immunity to top officials in exchange for research.

For years after World War II , in most of what was considered the "civilized" world, the truth behind Japanese Imperial Army Unit 731 was quietly swept away. Facts were suppressed. Memories questioned. Reports denied.

Even today, the true extent of Unit 731's wartime actions — horrendous, deadly medical experiments and lethal biological weapons testing on unsuspecting Chinese civilians — is known largely only to historians and scholars.

But the facts are out there for those who seek them. And for those who seek to use them for their own personal reasons.

"I think that it has become a piece of this tortured dialogue over the war between Japan and China. The Chinese have seized upon this quite a bit. And the Japanese right, the nationalist right, their basic view is that, 'Oh, the Chinese. This is all political.' ... And there is a certain truth to that," says Daniel Sneider , a lecturer in international policy at Stanford's Freeman Spogli Institute for International Studies . "There is a 'uses of the past' question here. Perhaps you could say it's cynical in that everybody does it."

The truth is that Japan's Unit 731 committed some of the most heinous war crimes ever. Thousands of prisoners were killed in cruel human experiments at Unit 731, which was based near the northeastern China city of Harbin, north of the Korean peninsula and on a border with Russia. Perhaps hundreds of thousands more — maybe as many as a half-million — were killed when the Japanese tested their biological weapons on Chinese civilians.

The exact number of dead is not known. It may never be known.

"It's very difficult to calculate," says Yue-Him Tam , a history professor at Minnesota's Macalester College and co-author of a book entitled, " Unit 731: Laboratory of the Devil, Auschwitz of the East (Japanese Biological Warfare in China 1933-1945) ." Tam, born and raised in China, has taught a class at Macalester on war crimes and memory in contemporary East Asia for more than 20 years. "If you include those victims who suffered from the other activities — not necessarily just used as human guinea pigs — the bombs in China ... it's very difficult to calculate."

The Start of Unit 731

America's shameful part, dealing with unit 731 today.

Unit 731 — its official name was the Epidemic Prevention and Water Purification Department of the Kwantung Army — was formed before World War II began (at least for the U.S., which didn't officially enter the war until December 1941). It came about sometime in the mid-1930s when Japan and China went to war , a conflict that eventually morphed into World War II's war in the Pacific theater.

The Unit's charge was clear from the beginning: testing, producing and storing biological weapons. Such activities were outlawed by at least two international treaties at the time, though the Japanese did not ratify the 1925 Geneva protocol . It didn't matter.

From the start, Unit 731, under General Shirō Ishii , was merciless.

Among the thousands of experiments conducted on prisoners: vivisections without anesthesia; injections of venereal diseases to examine their spread; amputations to study blood loss; removal of other body parts and organs; starvation; and deliberate exposure to freezing temperatures to examine the effects of frostbite . From a 1995 article in The New York Times , relating a story from a medical assistant in Unit 731:

Reportedly, not one of the thousands of prisoners that were experimented on — most of whom were Chinese, though many were Russian or Korean — survived.

Later, the Japanese took especially virulent forms of the plague and other pathogens that were developed at Unit 731, put them in canisters and dropped them on nearby towns to see if their weapons would work. They did.

Thousands of these still-dangerous bombs remain in the Chinese countryside today, Tam says. Some people still suffer from the Japanese "dirty" bombs.

At one time, the Japanese hatched a plan to infect fleas with a plague manufactured at Unit 731 and drop flea-filled bombs, launched from planes stored aboard submarines, on San Diego in a mission code-named Operations Cherry Blossoms at Night . The war ended before the plan could be executed.

After the U.S. dropped atomic bombs on Japan and effectively ended the war in 1945, Japanese leaders ordered the destruction of Unit 731, which included more than 150 buildings and two airports. As the victorious Allied forces approached, many hundreds of remaining prisoners were killed. The thousands of people who worked in the place and conducted experiments on healthy, living humans scattered, many never to face justice.

The top doctors and soldiers at Unit 731 kept careful records of their experiments, and used them to leverage their way to freedom after the war. When the Allies swept into China, they agreed to grant Ishii and many of his associates immunity from prosecution for war crimes . The reasons: The U.S. wanted Unit 731's research for its own use , and it wanted to keep that information out of the hands of others, including the Russians. Thus, for years, the true nature of what went on in Unit 731 was shielded from public knowledge.

Some of the truth came out in the Khabarovsk War Crimes Trial , held in that Russian city in December 1949. Twelve members of Unit 731 and associated units were tried. All were found guilty and imprisoned. Despite that trial, though, much of what went on in Harbin was immediately classified by the U.S. government and remained clouded in secrecy.

More details about Unit 731 are still being unearthed. A confession from a unit commander, written to U.S. interrogators at a base in Maryland shortly after the war, was released in August 2021 by a Chinese provincial agency. Chinese and Russian news outlets heralded the release, which highlighted America's part in using the information gathered by Unit 731, hiding it and protecting its sources from further prosecution.

"The United States is not the outsider to this. Previously, I think the tendency was for the people in the United States to think, 'This is a problem between Japan and its neighbors.' But not only were we of course the major combatant in the war, we shaped the postwar settlement, including decisions like the one concerning Unit 731," Sneider says. "We made the big decisions about what was a war crime and what wasn't ... We're the creator of the postwar order, and therefore we have responsibility and involvement in dealing with the issues that were left, unfortunately, unresolved."

Research on 731 continues to be conducted all over the world. As recently as 2018, the Japanese government provided a list of more than 3,600 members of Unit 731 to a Japanese scholar. Yet even with more information, with politicians and the governments of various countries opening their records, the facts remain largely in the shadows and in some dispute.

In China, with the resurgent government now not as dependent on Japan as it was in the years following World War II, the Chinese are demanding more answers, eager to hold old rival Japan responsible.

For their part, most Japanese are not nearly as willing to engage in discussions about what is considered by many Japanese as a shameful period in that country's history. Some say the surge in Chinese interest in Unit 731 is nothing more than political in nature.

The U.S. is dealing with its own internal demons about its history with Unit 731.

These varying viewpoints, and others in the region and throughout the world, complicate matters. From 2006 to 2016, Sneider and others at Stanford's Walter H. Shorenstein Asia-Pacific Research Center conducted a project, " Divided Memories and Reconciliation ," aimed at examining the historical memory of the wartime period in Asia. These histories, viewed differently, go directly to ideas about national identity and nationalism. They are tricky histories to examine, uncovering differences that often stay unresolved.

"Sometimes the truth is pretty elusive," says Sneider. "To some degree, the goal is not necessarily always to establish 'the fact.' That's a good goal, but it may not be possible. The goal, if you're seeking reconciliation, the goal may be to understand the different perceptions of the other.

"In Japan, wartime memory is highly contested within Japan. They've been battling over these issues since 1945. Sometimes it's important just for Koreans and Chinese and Americans to understand what's going on within Japan. That path is contrived; to try to get to reconciliation by agreeing on what happened."

People may not agree on how many people were killed by the criminals in Unit 731, who did it, how it was done, or why it occurred. They can, and should, look critically upon America's decisions after the war, too.

But this much is indisputable: What happened in Unit 731 was an abomination.

In August 2015, The Museum of Evidence of War Crimes by Japanese Army Unit 731 opened in an area just south of Harbin, a city of more than 5 million people. Tam is among the millions who have visited the site.

"The room where they experimented with poisonous gas, there are still walls standing there, and the walls are really thick, almost like 1-meter [3.2-feet] thick, to prevent leaking of something. When I saw these things, I was really shedding tears as to how people can do that," Tam says. "It was very moving.

"I am a historian. The most important thing that matters to me is the facts. I want to find out the facts. And that was a fact, Unit 731. The crimes they committed and produced are facts."

Japanese war criminals were tried at the International Military Tribunal for the Far East , otherwise known as the Tokyo War Crimes trial. Testimony from Ishii , gathered in Maryland after his postwar arrest, was used in the trial, too. But Ishii, the architect behind the Unit 731 atrocities, was never charged. He died in Tokyo in 1959, a free man.

Frequently Asked Questions

How did unit 731's activities impact international laws on human experimentation, what were the long-term health effects on survivors of unit 731's experiments.

Please copy/paste the following text to properly cite this HowStuffWorks.com article:

National Museum of Nuclear Science & History

• World War II History

By Avani Sihra

In the 1930s-‘40s, the Japanese Empire committed atrocities across Asia, such as the Rape of Nanking. German crimes such as human medical testing committed in concentration camps tend to receive more attention than Japan’s crimes against humanity, as more research has been done and more historians have spent time looking back and studying these horrific acts. However, the Japanese too played a part in human medical testing in a secret project called Unit 731.

Begun in 1937, Unit 731, located in Harbin, China, was created with legitimate intentions by the Japanese government. Started as an agency to promote public health, Unit 731 was meant to conduct research that would benefit Japanese soldiers, such as learning more about the ways in which the human body can withstand hunger and thirst and fight diseases. Early experiments were conducted on volunteers who had signed consent waivers, giving personnel permission. However, as the war intensified, they changed their methods.

Although the 1925 Geneva Accords had banned the use of biological or chemical weapons in warfare, the Japanese nevertheless wanted to prepare for these types of warfare. As these types of experiments were naturally ones that most people would not volunteer to take part in, the Japanese decided to use prisoners of war as their test subjects. Unit 731’s victims who were primarily Chinese and Russians, along with some Mongolians and Koreans.

The leader of the unit was Lieutenant General Shiro Ishii. Along with the other scientists he recruited, they experimented by infecting test subjects with different types of diseases to see how their bodies would respond to pathogens. As the Japanese destroyed most of the Unit’s records at the end of the war, little is known about the scientists who worked there.

Using the test subjects, the scientists injected different germs to see how they would react to one another in the human body, in an attempt to create new diseases. Referring to their victims as Maturas , or “wooden logs,” Japanese scientists would perform different types of procedures, such as vivisection, on live victims. Rats infected with the bubonic plague were released onto victims, with the intention of infecting the subjects so that they could be studied. Unit 731 was a place of torture that was, in the minds of many Unit 731 workers, a necessity in order to win the war .

Scientists in Unit 731 also experimented on their test subjects through pregnancy and rape. Male prisoners infected with syphilis would be told to rape female prisoners as well as male prisoners in order to see how syphilis spreads in the body. Women were involuntarily impregnated and then experiments were done on them to see how it affected the mother as well as the fetus. Sometimes the mother would be vivisected in order to see how the fetus was developing. 

Once it was clear that the Japanese were going to lose the war, unit workers destroyed much of the evidence of the experiments. Upon the formal surrender of the Japanese in August 1945, Unit 731 was officially terminated. The Japanese government did not admit to the wrongdoing committed by Unit 731 until very recently. The government did not acknowledge the atrocity until 1988, and even then, they did not apologize for what had happened. The project was highly secretive and much of the evidence had been destroyed; in addition, government officials who were aware of what happened in Unit 731 did not make their knowledge known to the public. Because of this lack of acknowledgment, the Chinese government took it upon themselves to spread awareness of the atrocities. In 1982, they established a museum in the same place where Unit 731 operated during the war.

Unlike some of the Nazi doctors who conducted experiments on prisoners and concentration camp inmates, none of those involved with the experiments at Unit 731 were ever punished for their crimes. Instead, after war’s end, many re-entered society and went on to have very successful careers in their fields. American forces , chiefly General Douglas MacArthur, decided not to put workers of Unit 731 on trial. MacArthur granted those involved immunity in exchange for the information they had gathered while doing their experiments. He believed that pursuing trials against these people would get in the way of the Americans receiving the medical information that had been documented from these experiments. Because of this decision, justice was never served.

Frank, Richard B. Downfall. Penguin Books, 1994.

Kristof, Nicholas D. “Unmasking Horror — A special report; Japan Confronting Gruesome War Atrocity.” New York Times, https://www.nytimes.com/1995/03/17/world/unmasking-horror-a-special-report-japan-confronting-gruesome-war-atrocity.html . Accessed 3 May 2018.

Stockton, Richard. “Inside Unit 731, World War II Japan’s Sickening Human Experiments Program.” All That’s Interesting, http://allthatsinteresting.com/unit-731 . Accessed 3 May 2018.

Unit 731. Unit 731: Japan’s Biological Warfare Project, https://unit731.org/ . Accessed 3 May 2018.

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Japan's Biological Warfare Project

• Experiments
• Harbin museum
• Yoshio Shinozuka
• Documentaries

Most of us heard about the horrible experiments on humans of the Nazis done by doctor Mengele. But the Nazis weren’t alone in conducting cruel experiments on humans.

One of the lesser known atrocities of the 20th century was committed by the Imperial Japanese Army’s Unit 731. Some of the details of this unit’s activities are still uncovered.

This webpage was set up to collect and organize the information known to date about Unit 731 and present it to anyone interested.

For 40 years, the horrific activities of “Unit 731” remained one the most closely guarded secrets of World War II. It was not until 1984 that Japan acknowledged what it had long denied – vile experiments on humans conducted by the unit in preparation for germ warfare.

Deliberately infected with plague, anthrax, cholera and other pathogens, an estimated 3,000 of enemy soldiers and civilians were used as guinea pigs. Some of the more horrific experiments included vivisection without anesthesia and pressure chambers to see how much a human could take before his eyes popped out.

Unit 731 was set up in 1938 in Japanese-occupied China with the aim of developing biological weapons. It also operated a secret research and experimental school in Shinjuku, central Tokyo. Its head was Lieutenant Shiro Ishii.

The unit was supported by Japanese universities and medical schools which supplied doctors and research staff. The picture now emerging about its activities is horrifying.

According to reports never officially admitted by the Japanese authorities, the unit used thousands of Chinese and other Asian civilians and wartime prisoners as human guinea pigs to breed and develop killer diseases.

Many of the prisoners, who were murdered in the name of research, were used in hideous vivisection and other medical experiments, including barbaric trials to determine the effect of frostbite on the human body.

To ease the conscience of those involved, the prisoners were referred to not as people or patients but as “Maruta”, or wooden logs. Before Japan’s surrender, the site of the experiments was completely destroyed, so that no evidence is left.

Then, the remaining 400 prisoners were shot and employees of the unit had to swear secrecy. The mice kept in the laboratory were then released, which could have cost the lives of 30,000 people, since the mice were infected with the bubonic plague, and they spread the disease.

Few of those involved with Unit 731 have admitted their guilt.

Some caught in China at the end of the war were arrested and detained, but only a handful of them were prosecuted for war crimes.

In Japan, not one was brought to justice. In a secret deal, the post-war American administration gave them immunity for prosecution in return for details of their experiments.

Some of the worst criminals, including Hisato Yoshimura, who was in charge of the frostbite experiments, went on to occupy key medical and other posts in public and private sectors.