Unit 731

Unit 731 was a secret biological and chemical warfare research unit run by the Imperial Japanese Army from 1933 to 1945. Officially called the Manchu Detachment 731, it was also known as the Kamo Detachment and the Ishii Unit.

The main facility was in the Pingfang district of Harbin, in the Japanese puppet state of Manchukuo, which is now Northeast China. From there, the unit ran several branches across mainland China and Southeast Asia under the "Epidemic Prevention Department." During the Second Sino-Japanese War, the unit carried out the largest biological weapons attacks in history. These attacks poisoned water supplies, food, and crops, and they killed an estimated 200,000 people in various cities and villages.

The unit was led by General Shirō Ishii and received strong support from the Japanese military. An estimated 14,000 people were killed by human experimentation at the facility. The prisoners, whom staff called "logs" (or "maruta"), were mainly Chinese civilians. There were also Russians, Koreans, and others, including children and pregnant women. Experiments included infecting prisoners with deadly diseases, vivisection, organ harvesting, testing hypobaric chambers, and exposing victims to chemical agents and explosives. No documented survivors are currently known.

After the war, twelve Unit 731 members were tried by the USSR in the 1949 Khabarovsk war crimes trials and received relatively lenient prison sentences. Ishii and other members avoided prosecution by the United States, which was interested in their data. American officials were also concerned that a war crimes trial would expose that data to the USSR and force the U.S. to reveal its own biological warfare abilities. The Americans granted them immunity in exchange for their research data, which was later found to be of little value, since the American biological warfare program was already more advanced than Japan's. The Truman administration hid the unit's crimes and paid stipends to former personnel.

On 28 August 2002, the Tokyo District Court acknowledged that Japan had conducted biological warfare in China and held the state responsible for related deaths. However, it rejected compensation, arguing that postwar treaties such as the Treaty of San Francisco had already provided for it. In April 2018, the National Archives of Japan released a near-complete list of 3,607 personnel connected to Unit 731. The Japanese government has never officially acknowledged its biological warfare.

Formations

The Empire of Japan began its biological weapons program in the 1930s. The Geneva Protocol of 1925 banned these weapons in fights between countries, and Japan's leaders reasoned that the ban showed the weapons would work. After the Japanese invasion of Manchuria began in 1931, Japan built Unit 731 there. Manchuria kept the research station away from Japan's islands and gave Japan access to many Chinese people to use as test subjects. Most victims were Chinese, but many were of other nationalities. The facilities included a prison area for holding victims, and the research rooms were built around it. Founded in 1933, Unit 731 grew to 3,000 staff members, 150 buildings, and space to hold up to 600 prisoners at once. Unit 731 was a secret division of the Kwantung Army in Manchuria. It was led by Lieutenant General Shirō Ishii, and its network was based at a laboratory in Tokyo, set up in 1932. In 1936, Emperor Hirohito approved the unit's expansion into the Kwantung Army, with a base in Xinjing. One of Ishii's main supporters was Colonel Chikahiko Koizumi, who later served as Japan's Health Minister from 1941 to 1945.

Main article: Epidemic Prevention and Water Purification Department

Unit 731 first operated at the Zhongma Fortress, a prison and experimentation camp in Beiyinhe, about 100 kilometers (62 mi) south of Harbin along the South Manchuria Railway. Its prisoners included criminals, captured bandits, anti-Japanese partisans, political prisoners, and people falsely accused by the Kempeitai. Prisoners were fed well at first, with rice or wheat, meat, fish, and sometimes alcohol, so they would be healthy when experiments began. Some were infected with plague and other microbes. A prison break in 1934 and an explosion in 1935 that was believed to be sabotage led Ishii to close the fortress and move to Pingfang, near Harbin, to build a much larger facility. The decree of 1936 also created the Kwantung Army Military Horse Epidemic Prevention Workshop, later called Unit 100, and the Kwantung Army Technical Testing Department, later called Unit 516. After the Japanese invasion of China in 1937, sister units were set up in major Chinese cities, including Unit 1855 in Beijing, Unit Ei 1644 in Nanjing, and Unit 8604 in Guangzhou. Later units were added in Singapore, Malaya (now Malaysia), Indonesia, the Philippines, Papua New Guinea, Thailand, and Burma, with Unit 9420 in Singapore. At its height in 1939, Ishii's network oversaw more than 10,000 personnel. Doctors and professors from Japan were drawn to Unit 731 by the chance to conduct human experiments and by the Army's strong financial support.

Experiments

Unit 731 carried out many experiments on prisoners, mostly Chinese prisoners of war and civilians. Military police and a special agency found people to use as test subjects, and a group of doctors looked after them until they were sent for experiments. Many were deliberately infected with diseases. The unit also tested biological weapons and chemical weapons on prisoners and studied the effects of extreme cold, heat, and starvation. Prisoners who did not die from infection were usually killed so their bodies could be studied. Biological attacks on Chinese cities spread bubonic plague and other diseases, including in Changde and Nanjing. These diseases were estimated to have killed at least 400,000 Chinese civilians.

The prisoners were treated as objects. Staff used the word "logs" for them, a term that began as a joke. A former member of the unit later said that some experiments had no medical purpose and were "just playing around." Unit 731 is remembered as one of the most cruel programs of the war, and its story shows how important it is to treat all people with respect.

Prisoners and victims

After Japan's defeat in World War II, the Japanese murdered every prisoner in the unit, and the remains were cremated and buried on the Unit 731 grounds. At least 3,000 men, women, and children were used in experiments at the Pingfang camp alone, not counting victims from other sites such as Unit 100. Most of the victims were Chinese, with smaller numbers of Russian, Mongolian, and Korean people. The test subjects included common criminals, captured bandits, anti-Japanese partisans, political prisoners, homeless people, and mentally disabled people, including infants, men, the elderly, and pregnant women. No Unit 731 prisoners survived, and at least two escape attempts failed.

In 2002, a symposium in Changde, China, estimated that around 580,000 people were killed by the Imperial Japanese Army through germ warfare and other human experiments. The American historian Sheldon H. Harris states that over 200,000 people died.

Personnel and postwar accountability

Unit 731 included hundreds of military and civilian personnel who took part in wartime human experimentation, biological weapons development, and chemical warfare. After Japan's surrender, several members were held at the Fushun War Criminals Management Centre and the Taiyuan War Criminals Management Centre. Many were later sent back to Japan, where some joined the Association of Returnees from China and spoke about their activities. Prominent members included Lieutenant General Shirō Ishii, the overall commander; Major General Masaji Kitano, commander from 1942 to 1945; Lieutenant Colonel Ryōichi Naitō, the founder of Green Cross; Prince Tsuneyoshi Takeda; Prince Naruhiko Higashikuni; Yoshio Shinozuka, who later gave testimony; Yasuji Kaneko, who also later gave testimony; Shigeru Fujita; and Ken Yuasa. Civilian employees and students included Shigeo Ozeki, Kioyashi Mineoi, Masateru Saitō, and Yoshio Furuichi.

In 1949, the Soviet Union tried twelve former members of Unit 731 and related units during the Khabarovsk war crimes trials. The table below lists their ranks, roles, and sentences.

In April 2018, the National Archives of Japan released a nearly complete list of 3,607 people linked to Unit 731 to Katsuo Nishiyama, a professor at Shiga University of Medical Science. Nishiyama said he planned to publish the list online to encourage more research. In August 2023, Seiya Matsuno of Meiji Gakuin University found a more detailed staff listing in the National Archives.

Divisions

Unit 731 was divided into eight divisions:

• Division 1 researched bubonic plague, cholera, anthrax, typhoid, and tuberculosis using living people. A prison was built to hold around three to four hundred people for this purpose.
• Division 2 researched biological weapons for use in the field, especially devices that spread germs and parasites.
• Division 3 made shells that contained biological agents. It was stationed in Harbin.
• Division 4 mass-produced and stored bacteria.
• Division 5 trained personnel.
• Divisions 6–8 were equipment, medical, and administrative units.

Facilities

Main article: Epidemic Prevention and Water Purification Department

Unit 731 had other units under it in the chain of command, and several other units belonged to Japan's biological weapons programs. Most or all units had branch offices, which were also often called "Units." The name Unit 731 can refer to the Harbin complex, or to the whole organization and its branches. The Harbin complex covered six square kilometers (2.3 sq mi) and had more than 150 buildings, designed to be hard to destroy by bombing. It held around 4,500 containers for raising fleas, six cauldrons for making chemicals, and around 1,800 containers for making biological agents. About 30 kilograms (66 lb) of bubonic plague bacteria could be produced in a few days. Some of its branch facilities are still used by Chinese industrial companies, and part of the site has been preserved as a museum open to visitors.

Unit 731 had branches in Linkou (Branch 162), Mudanjiang, Hailin (Branch 643), Sunwu (Branch 673) and Hailar (Branch 543). A medical school and research facility belonging to Unit 731 operated in the Shinjuku District of Tokyo during World War II. In 2006, Toyo Ishii, a nurse who had worked at the school during the war, revealed that shortly after Japan's surrender in 1945 she had helped bury human remains on the school grounds. In response, the Ministry of Health began digging up the site in February 2011. Tokyo courts acknowledged in 2002 that Unit 731 has been involved in biological warfare research. As of 2011, however, the Japanese government had not officially acknowledged the harm done to test subjects, and it rejected Chinese government requests for DNA samples that could be used to identify human remains found near an army medical school.

Surrender and immunity

As the Second World War came to an end, the prisoners held at the compound were killed to hide the evidence, and no survivors were recorded. When the Red Army arrived in August 1945, the unit had to leave its work in a hurry. Officials in Tokyo ordered the destruction of all incriminating materials, including those at Pingfang. Some Japanese troops blew up the compound in the final days of the war, but many parts stayed somewhat intact. After Japan's 1945 surrender, Lieutenant Colonel Murray Sanders, a highly regarded microbiologist, arrived to investigate Japanese biological warfare. Little information was shared until he threatened to bring in the Soviets, because the Japanese wanted to avoid prosecution under the Soviet legal system. Sanders took the information to General Douglas MacArthur, the Supreme Commander of the Allied Powers, who secretly granted immunity to the unit's doctors, including its leader Shirō Ishii, in exchange for American access to their research.

The Tokyo War Crimes Tribunal heard only one brief reference to the experiments, and William Webb dismissed it for lack of evidence. While German physicians were brought to trial, the United States kept Japan's information hidden. In 1948 an American court in Yokohama tried Japanese doctors for experiments on captured American pilots, and two were sentenced to death. The Soviet Union prosecuted 12 top military leaders and scientists from Unit 731, Unit 1644 in Nanjing, and Unit 100 in Changchun in the Khabarovsk war crimes trials. The lead prosecutor, Lev Smirnov, had also worked at the Nuremberg Trials. The defendants received sentences of 2 to 25 years in a Siberian labor camp, and all of them returned to Japan by 1956. The United States refused to recognize these trials. On 6 May 1947, MacArthur, the Supreme Commander of the Allied Forces, wrote to Washington that more information could be gained if Japanese participants were told it would be kept in intelligence channels and not used as war crimes evidence.

In popular culture

Unit 731 has appeared in many books and comics. Crisis in the Ashes (1999), a novel by William W. Johnstone, features the grandson of a Unit 731 scientist who uses plague as a weapon. The Man Who Ended History: A Documentary (2011), a novella by Ken Liu, tells of an inventor of time travel who uses the invention to draw attention to the atrocities of Unit 731. In My Hero Academia (2014–2024), a manga by Kōhei Horikoshi, a character originally named "Shiga Maruta" caused backlash for referencing Unit 731's human test subjects, and the name was later changed. The Solomon Curse (2015), a novel by Clive Cussler and Russell Blake, includes a subplot about secret human experimentation linked to Unit 731. Tricky Twenty-Two (2015), a novel by Janet Evanovich, features a villainous biology professor trying to recreate Unit 731's ways of spreading plague. The Collector – Unit 731 (2021), a comic miniseries by Dark Horse Comics, written by Rod Monteiro and illustrated by Will Conrad, follows a prisoner of war who was captured and experimented on by Unit 731.

Unit 731 has also been shown on television. Unit 731 – Did the Emperor Know? (1985), a documentary by Television South, investigates what Imperial Japan knew about Unit 731. 731 (Chinese: 七三一) (2015), a five-part CCTV documentary, chronicles Unit 731's crimes and its legacy after the war. The Truth of Unit 731: Elite Medical Students and Human Experiments (2017), an NHK documentary, features interviews and recovered records from Unit 731 personnel. Gyeongseong Creature (2023–2024), a South Korean historical horror series, was inspired by the war crimes of Unit 731 in its depiction of biological warfare research and human experimentation under Japanese occupation. 731 (2025), a Chinese movie, follows prisoners' experiences in Unit 731 in occupied Northeastern China.