Unit 731 was the secret biological warfare unit set up in the northeast of China following the Japanese invasion; the headquarters were on the outskirts of Harbin in Manchukuo. Unit 731 researched, developed, produced, and tested biological weapons. As part of its research program, it experimented on humans and animals. The details of Unit 731’s activities remained largely unknown until the mid-1980s, when a number of documents concerning its activities came to light. Many of these documents were produced by U.S. military organizations, such as G-2 (Intelligence) in the Office of the Assistant Chief of Staff and the Office of the Judge Advocate General. Substantial parts of these records were information seized by the U.S. occupation forces directly from former members of Unit 731 after the war, but these were never disclosed to the public.

In 1931 Japanese forces invaded the northeast of China, claiming that Chinese forces had destroyed the railway at Lake Liu near Mukden in southern Manchuria, although this had actually been done by the Japanese themselves to provide a pretext for the invasion. This marked the beginning of the so-called Manchurian incident. In 1932 the Japanese government annexed the northeast of China and set up the Manchukuo puppet state. In reality, Manchukuo was a Japanese colony and was governed by the Kwantung Army, the most powerful of the Japanese forces.

Ishii Shiro, a prominent physician and a graduate of Kyoto University, traveled to Europe in 1928 to investigate the situation concerning biological weapon. When Ishii returned to Japan in spring 1930, he urged the military leaders to provide a means for researching biological warfare and developing the capability to wage it. At that time, various Western nations were actively involved in research on biological weapons, although the United States had not yet started it. In 1932 Ishii set up the Epidemic Prevention Laboratory within the military medical school in Tokyo with the full support of the military. At the same time, Ishii set up in Manchukuo a small and secret subgroup, the Togo Unit, in the village of Bei-inho, 100 kilometers southeast of Harbin. Remoto Manchuduo was chosen primarily because researchers wanted to conduct medical experiments on human using Chinese prisoners began as soon as the Togo Unit was established. Thus, research on defensive methods against biological weapons as conducted mainly in Tokyo, and research on offensive use and actual production of such weapons was carried out in Manchukuo.

In 1925 the Geneva Convention prohibited the use of chemical and bacteriological weapons. Ishii obviously knew that his plans contravened the convention, but he also knew how effective biological weapons could be. The Ishii group sought out all bacteria and viruses that could prove useful as weapons and for which vaccines could be developed so as to protect the Japanese forces using them.

In 1936 the Togo Unit was reorganized and expanded into the Epidemic Prevention Department of the Kwantung Army (the Ishii Unit). A smaller section (the Wakamatsu Unit) concerned with combating animal diseases was set up by the Kwantung Army at Xinjing. Both units were set up with the approval of Japanese Imperial Headquarters. In 1938 a special military zone was declared at Pingfan, 25 kilometers southeast of Harbin, and the local residents were all evicted. Construction of a huge facility for the production of biological weapons began. On August 1, 1940, the Ishii Unit was renamed the Epidemic Prevention and Water Purification Department of the Kwantung Army (a description the very opposite of its real aims), although after 1941 it was more commonly referred to as Manchukuo Unit 731. Unit 731 was composed of four sections: research, experiments, antiepidemic, and water purification and productions.

After Unit 731 was set up in Pingfan, many faculty members of the military medical school were sent to Manchukuo and became involved in experimenting on humans to develop biological weapons.  In fact, Ishii started recruiting young elite medical specialists from various Japanese universities a few years before the establishment of Unit 731 in 1936.  Professors in the medical school of Kyoto University in particular assisted Ishii with his recruitment. Branch units were set up in Beijing, Nanjing, Guangdong, and Spingapore; there units conducted experiments on weapons developed by Unit 731 and made plans for waging biological warfare within those regions.  At this time Colonel Ishii had 3,000 staff in Unit 731 and as many as 20,000 staff under his command if all members from the branch units were totaled.

Various methods were developed for dispersal of biological weapons.  One was to introduce the pathogen to a local water supply or food supply.  Another was to use airborne means, and Unit 731 developed a bomb specifically designed for dispersing pathogens from aircraft.  In 1939, when Japanese and Russian force, Unit 731 introduce the typhoid-fever pathogen into river in the area.  In 1940 and 1941 the unit used aircraft to spread cotton and rice husks contaminated with the black plague at Changde and Ningbo, in central China.  About 100 people died from the black plague in Ningbo as a result.  From the viewpoint of the Japanese, the casualties at Ningbo were insufficient, so they developed a bomb enabling more efficient dispersal from greater heights (thus making the process less hazardous for air crews, who would be subject to antiaircraft fire if required to fly low over an ear in order to deliver their payload.) This bomb was not widely used, however, as it was not perfected until close to the end of the war.

Unit 731 regarded fleas as the most useful vector for pathogens, especially the plague.  The unit bred massive members of fleas and rats for producing the plague bacillus and tested whether fleas could survive being released from bombs dropped from aircraft.  The unit also developed anthrax-bacillus bombs, which proved successful because the bacillus is heat resistant. Shrapnel from the bombs carrying the bacillus was highly efficient at infecting those hit by it.  The anthrax bomb was tested many times on humans at Anta, 146 kilometers from Pingfan.

After the outbreak of World War II, the Japanese continued to use biological weapons against the Chinese.  They sprayed cholera, typhoid, plague, and dysentery pathogens in the Jinhua area of Zhejian province in June and July 1942.  The was done in retaliation for the first U.S. air raids on mainland Japan, in which Tokyo and Nagoya were bombed. After these raids, the Allied aircraft landed at airfields in China, and the Japanese took this as Chinese collaboration with the Allies.  In the Jinhua pathogen attack, however, the Japanese also fell victim to the diseases, and large numbers of Japanese casualties occurred.  According to one source, over 1,700 Japanese soldiers died.

It is well known that Unit 731 used large number of Chinese people for experiments.  Many Chinese who rebelled against the Japanese occupation were arrested and sent to Pingfan where they became guinea pigs for Unit 731; there is evidence that some Russian prisoners were also victims. The prisoners subjected to experiments were called 'maruta' (literally 'logs') by the Japanese. Every year the military police rounded up approximately 600 maruta send to Pingfan.  After succumbing to the disease, the prisoners were usually dissected, and their bodies were then cremated within the compound.

Unit 731 also conducted frostbite experiments on the maruta. Frostbite was a severe problem for the Japanese forces in Manchukuo, where the winters are extremely cold. The prisoners were tied up outdoors in temperatures as cold as -20 degrees Celsius and parts of their bodies were sprayed with salt water in order to induce frostbite.  Their arms were hit with hammers to determine whether they were frostbitten.  They were then immersed in hot water of ranging temperatures in orders to determine how recovery from frostbite could best be facilitated. In extreme cases, the prisoners' skin and muscles sloughed off in response to this treatment and the victims died immediately. As a result of the experiments, it was found the immersing frostbitten limbs in body-temperature water best facilitated recovery. It is said that General Ishii and his colleagues were particularly proud of this discovery.

Maruta also were subjected to poisonous gas experiments. In one experiment conducted September 7-10, 1940, 16 Chinese prisoners were exposed to mustard gas in a simulated battle situation that employed a macabre from of experimental manipulation. The prisoners were positioned in various places, such as under a machine-gun cover or inside a building, and mustard gas shells were fired toward them. Some of the prisoners had gas masks and others did not, and they were also dressed in different types of clothes. Every few hours after firings the condition of the prisoners was monitored. In another experiment, five prisoners were forced to drink a liquid from of mustard gas and their condition was then monitored for a five-day period.

Most Japanese citizens were unaware of the unit's activities until 1981, when author Seiichi Morimura exposed the unit's dark history in a book, "The Devil's Gluttony". Many of the unit's doctors and researchers became heads of medical and pharmaceutical firms in post-war Japan.

The Japanese government has never formally apologized for Unit 731's activities, and did not even admit to its existence until August 1998, when the Supreme Court ruled that the existence of the unit was accepted in academic circles.

In 1995, families of Chinese victims filed a lawsuit demanding the Japanese government pay compensation of 100 million yen (US$826,000)

Germ warfare timeline

1925 -- Geneva Convention governing wartime conduct bans biological weapons. Japan refuses to approve treaty.

1932 -- Japanese troops invade Manchuria. Shiro Ishii, a physician and army officer who was intrigued by germ warfare, begins preliminary experiments.

1936 -- Unit 731, a biological-warfare unit disguised as a water-purification unit, is formed. Ishii builds huge compound -- more than 150 buildings over six square kilometers -- outside the city of Harbin. Some 9,000 test subjects, which Ishii and his peers called ''logs,'' eventually die at the compound.

1942 -- Ishii begins field tests of germ warfare on Chinese soldiers and civilians. Tens of thousands die of bubonic plague, cholera, anthrax and other diseases. U.S. soldiers captured in Philippines are sent to Manchuria.

1945 -- Japanese troops blow up the headquarters of Unit 731 in final days of Pacific war. Ishii orders 150 remaining ''logs'' killed to cover up their experimentation. Gen. Douglas MacArthur is named commander of the Allied powers in Japan.

1946 -- U.S. coverup of secret deal with Ishii and Unit 731 leaders -- germ warfare data based on human experimentation in exchange for immunity from war-crimes prosecution -- begins in earnest. Deal is concluded two years later.

1981 -- John Powell, a former publisher of a Shanghai magazine who was unsuccessfully tried for sedition in the early 1950s after accusing the United States of using germ warfare in Korea, exposes immunity deal in the Bulletin of Atomic Scientists.

1985 -- Dr. Murray Sanders, a former lieutenant colonel who was a U.S. adviser on biological warfare, claims that he persuaded MacArthur to approve the immunity deal in the fall of 1945.

1986 -- Congressional subcommittee holds one-day hearing in Washington, called by Rep. Pat Williams of Montana, aimed at determining whether U.S. prisoners of war in Manchuria were victims of germ-warfare experimentation. Hearing is inconclusive.

"I cut him open from the chest to the stomach and he screamed terribly and his face was all twisted in agony. He made this unimaginable sound, he was screaming so horribly. But then finally he stopped. This was all in a day's work for the surgeons, but it really left an impression on me because it was my first time."

Sheldon Harris, a historian at California State University, in Northridge, estimates that more than 200,000 Chinese were killed in germ warfare field experiments. 

A Unit 731 doctor vivisecting a pregnant girl who had his baby after being raped. 

Tokyo, a memorial tower for Unit 731

Sources

Tanaka, Yuki. Hidden Horrors 'Japanese War Crimes in World War II'
Sheldon H. Harris Factories of Death
Gavan Daws Prisoners of the Japanese: POWS of World War II in the Pacific