Developments in viral hepatitis have been traced from Saul Krugman's distinction of two types, MS1 and MS2 (A and B), and Baruch S. Blumberg's discovery of Australia antigen. Hepatitis A has been grown in tissue culture, the structure of the virus is known, and the acute disease can be diagnosed. Knowledge of the molecular biology of the more complex hepatitis B virion has allowed the development of an effective vaccine and distinction of replicative and nonreplicative stages of infection. Integration, in the hepatocyte, of hepatitis B viral DNA into host DNA is the precursor of liver cancer. The infection of hepatitis B carriers with another infectious agent, delta, has added a new dimension to the problem. Other unidentified causes of hepatitis have been lumped together as non-A, non-B, and these remain to be defined and accurately diagnosed.