Of six chimpanzees inoculated with materials positive for hepatitis B antigen (HB Ag), five developed evidence of hepatitis B virus (HBV) infection. In two of the five there was serologic, biochemical, and histologic evidence of type B viral hepatitis accompanying infection. HB Ag in the cytoplasm of hepatocytes and HBV-associated core antigen (HBC Ag) and virus-like particles in some hepatocyte nuclei were found by immunofluorescence and electron microscopy. Of the remaining three animals, two developed transient circulating HB Ag followed by antibody to HB Ag (HB Ab), and the third developed HB Ab in serum approximately three months after inoculation without any biochemical or histologic evidence of disease. Three of the five infected animals represented a second passage of HBV from a chimpanzee infected with plasma known to transmit type B viral hepatitis to man. Evidence of infection of one of these three animals included demonstration of the presence of the hepatitis B virus in a preparation of HB Ag that had been partially purified by isopycnic banding in a continuous sucrose gradient. Fluorescent staining of liver tissue from a chimpanzee shown to be a chronic carrier of HB Ag revealed HB Ag in the cytoplasm of most cells and HBC Ag rarely in cell nuclei; no virus-like particles in hepatocyte nuclei or liver histopathology were observed