Chimpanzees (Pan troglodytes ) became established as invaluable models for the study of human viral hepatitis after it was discovered, in 1967, that the chronic hepatitis B antigen carrier state existed in a naturally infected member of this species (1 –3 ). They were instrumental in the development of hepatitis B serologic marker assays and vaccines, and in safety testing human blood products for inactivation of this virus. In 1974, after hepatitis B-positive donor blood was no longer used, it was suggested that there were one or more unknown viral agents causing posttransfusion hepatitis (4 ,5 ) and chimpanzees were the most likely candidates for testing this hypothesis. By 1978 there were already three published reports that chimpanzees gave conclusive evidence of disease transmission after inoculation with putatively infectious materials from human patients with the newly named non-A, non-B hepatitis (NANBH) (6 –8 ).