Unit 731 was a secret military medical unit of the Imperial Japanese Army that researched biological warfare and other topics through human experimentation during the Sino-Japanese War (1937-1945) and World War II era. The unit was disguised as a water purification unit. It was based in Pingfan, near the city of Harbin in northeastern China, the region which was then part of the puppet state of Manchukuo. It is estimated that over 3,000 Chinese and Korean civilians and as well as Allied POWs were killed in the Unit 731 facilities. Certainly more people died of field experiments directed by Unit 731, but there is no well established number.
There were other units besides Unit 731, which serves as a general term in describing the Japanese biological warfare program.