Viral hepatitis, formerly a most elusive syndrome, is becoming a disease model that provides considerable insight into host-viral interactions. Identification of viral antigens and their respective antibodies, the use of powerful investigative tools (eg, radioimmunoassay, immunofluorescent microscopic techniques, and immune electron microscopy), and an emerging characterization of human immunology have combined to characterize the epidemiology, virology, and pathogenesis of the hepatitis type A and hepatitis type B viruses (HAV and HBV, respectively). These insights permit a rational approach to the diagnosis and treatment of these infections, suggest a role for the HBV in a number of immune complex disorders, and provide strong evidence that the HBV is a major cause of hepatocellular carcinoma. An unexpected result of the discovery of the agents of "infectious" (HAV) and "serum" (HBV) hepatitis was the unequivocal finding that at least two additional agents are responsible for the hepatitis syndromes in man. Further characterization of