Hepatitis B surface antigen (HBsAg) was identified with aldehyde fuchsin and immunoperoxida.se stain and by immunofluorescence in malignant hepatocytes with a ground-glass appearance in only one needle biopsy specimen of a series of biopsies from 130 consecutive cases of hepatocellular carcinoma. The patient was 14 years old. HBsAg was identified by aldehyde fuchsin stain in nonmalignant hepatocytes of 48 (58%) of 83 biopsy specimens that contained nonmalignant liver tissue. The antigen was demonstrable in significantly greater proportions of cases in younger age groups. A similar but not identical age relationship has been found for hepatitis B antigenemia in Hong Kong. It appears that the ability to produce HBsAg declines with age. The usual absence of demonstrable HBsAg in cells of hepatocellular carcinoma may be due to a failure of this characteristic to survive into the malignant cell line, and so does not invalidate the possibility that the hepatitis B virus (HBV) plays a direct role in the pathogenesis of hepatocellular carcinoma. In exceptional circumstances, as when hepatocellular carcinoma appears at an unusually early age, this marker is identifiable in cells of the tumor.