Arrested by the US occupation authorities at the end of World War II, Ishii and other Unit 731 leaders received immunity in 1946 from war-crimes prosecution before the Tokyo tribunal in exchange for germ warfare data based on human experimentation. On 6 May 1947, Douglas MacArthur wrote to Washington that "additional data, possibly some statements from Ishii probably can be obtained by informing Japanese involved that information will be retained in intelligence channels and will not be employed as 'War Crimes' evidence." The deal was concluded in 1948.
Ishii was never prosecuted for any war crimes. According to Richard Drayton, a Cambridge University history lecturer, Ishii later moved to Maryland where he conducted research into bioweapons. But according to Ishii's daughter Harumi, he stayed in Japan, where he died of throat cancer at the age of 67.
Skoro pola miliona žrtava i nikom ništa.