Plasma or serum from 4 patients with acute or chronic non-A, non-B post-transfusion hepatitis (P.T.H.) and from a blood-donor implicated in two cases of P.T.H. was inoculated into 5 chimpanzees. Biochemical and histological evidence of hepatitis developed in these 5 chimpanzees but not in a control animal. The mean incubation period in the chimpanzees was 13.4 weeks, compared with 7.7 weeks in the 4 patients with P.T.H. The peak alanine aminotransferase (A.L.T.) levels in the 5 chimpanzees were 265, 212, 219, 70, and 62 I.U./l. Histological changes ranged from mild to conspicuous hepatitis and generally correlated with the degree of A.L.T. elevation. There was no evidence of clinical disease and all animals went on to biochemical and histological recovery. There was no serological evidence of type A or type B hepatitis. Hepatitis was transmitted by serum derived from patients with chronic as well as acute hepatitis, strongly suggesting a chronic carrier state for the agent responsible for non-A, non-B hepatitis. Non-A, non-B hepatitis thus seems to be due to a transmissible agent which can persist and remain infectious for long periods.