Two human hypatitis viruses have been identified and characterized, but one or more additional agents exist. Hepatitis B virus (HBV) is a complex 42-nm predominantly double-stranded DNA virus with distinct surface and core antigens and an endogenous DNA polymerase. Hepatitis A virus (HAV) is a 27-nm RNA virus with enterovirus-like properties. Progressively more sensitive and specific immunologic assays have been applied to the study of viral hepatitis and are available for routine diagnostic purposes. As a result we recognize distinct serologic response patterns to infection, new antigenic markers, biochemical-biophysical characteristics of the viruses, and their epidemiologic features. Recombinant DNA technology has permitted the cloning of HBV genetic material and gene products in E. coli, but the virus has not been cultivated in vitro. In contrast, successful in vitro cultivation of HAV has finally been accomplished. Application of sensitive serologic tests for HAV and HBV has revealed that "non-A, non-B" agents account for a substantial proportion of transfusion-associated hepatitis as well as hepatitis occurring in the absence of percutaneous exposure. These agents have been transmitted to chimpanzees, and several putative virus antigen-antibody systems have been described; however, a specific association between these virus antigens and non-A, non-B hepatitis has not been established.