It has been suggested that cytotoxic T cells are involved in the recognition and lysis of the infected hepatocytes in chronic hepatitis B virus infection, and that the target antigen is probably HBcAg which is displayed on the hepatocyte membrane during active viral replication. However, studies in other viral infection have demonstrated that cytotoxic T cells recognize viral antigen on the infected cells only in the context of HLA class I antigens. To test whether this mechanism is also operative in chronic hepatitis B virus infection, we studied the expression of HLA class I antigens in livers from 35 patients with chronic hepatitis B virus infection by indirect immunofluorescence using monoclonal antibody against HLA class I antigens. The blocking effect of monoclonal antibody against HLA class I antigens on the in vitro T cell cytotoxicity to autologous hepatocytes was also studied. The results revealed that HLA class I antigen was undetectable on the hepatocyte membrane in all of 10 HBeAg-positive carriers with minor hepatitic activity, whereas it was demonstrated in 15 (88%) of the 17 HBeAg-positive patients with chronic active liver disease and in 7 (87%) of the 8 anti-HBe-positive “normal” carriers. The in vitro T cell cytotoxicity to autologous hepatocytes in six HBeAg-positive patients with chronic active liver disease was significantly inhibited by preincubation of hepatocytes with monoclonal antibody (10 to 40 μg per ml) against HLA class I antigen, but not by monoclonal antibody against HLA class II antigens and non-HLA-associated surface molecules (Leu 11). These findings suggested that HLA class I antigens became detectable on hepatocyte membrane while significant histological activity developed in chronic hepatitis B virus infection, and that the effector cytotoxic T cells required HLA class I antigen as target structure(s).