Routine use of commercially available antisera against hepatitis B core antigen(HBcAg) has permitted a reevaluation of the histochemical distribution of the antigen in liver tissue. HBcAg, classically described almost exclusively in the nucleus, was found with a very high frequency in the cytoplasm of liver cells as well. To elucidate the biologic significance of HBcAg expression and its relation to the natural course of hepatitis B virus(HBV) infection, the patterns of activity in 33 needle liver biopsies of HBsAg carriers were analysed. A good correlation of liver HBcAg with disease activity was demonstrated. HBcAg was present in the hepatocyte nuclei(nHBcAg) or cytoplasm(cHBcAg), or in both(mixed). Pure nHBcAg was seen mainly in non-aggressive reactive liver tissue and cHBcAg was predominantly associated with chronic active hepatitis(95%). The results suggest that expression of HBcAg correlates with the liver pathology and the possibility of HBcAg to be an immunological target for T cell mediated hepatocyte damage.