6 cases of non-A non-B hepatitis which followed administration of four different batches of concentrates of coagulation factor IX from commercial and non-commercial sources are described. Of 17 patients who received the concentrate on account of chronic liver disease, 4 developed hepatitis, and in 3 of these the illness proved fatal. The incubation periods ranged from 42 to 103 days (mean 65 days). 3 chimpanzees were inoculated with concentrate from the same batch used on the above patients, a further commercial batch upon which no adverse reactions had been reported, and plasma from a known non-A non-B carrier. All developed hepatitis after 10 weeks' incubation. Liver biopsy when serum-aminotransferase was at its highest level showed features consistent with acute hepatitis. As in the patients, viral markers for hepatitis A and B, cytomegalovirus, and Epstein-Barr virus were unchanged.