Non-A, non-B hepatitis was transmitted to four colony-born chimpanzees by intravenous inoculation of human sera. Two chimpanzees were inoculated with serum from a patient with a clinical and serological diagnosis of chronic non-A, non-B hepatitis whose blood appeared to transmit this disease to a nurse following accidental needle-stick, and the other two chimpanzees were inoculated with serum from either of two former blood-donors whose HBsAg-negative blood appeared to transmit clinically recognisable hepatitis, and who were found to have raised serum-aminotransferase levels 1 1/2 and 5 years later. Serum-aminotransferase levels rose in all four chimpanzees, beginning 2--4 weeks after inoculation: peak alanine-aminotransferase values were 210 to 328 I.U./l. Evidence of hepatitis was present in liver biopsy specimens from all four chimpanzees, beginning 8--10 weeks after inoculation. None showed serological evidence of infection with hepatitis A virus, hepatitis B virus, cytomegalovirus, or Epstein-Barr virus.