Hepatitis B virus (HBV) surface antigen (HBsAg) genes were injected directly into the liver of adult rats with non-histone chromosomal protein high mobility group 1 by the hemagglutinating B virus of Japan (Sendai virus)-liposome method (Kato, K., Nakanishi, M., Kaneda, Y., Uchida, T., and Okada, Y. (1991) J. Biol. Chem. 266, 3361-3364). Immunohistochemical analysis showed that HBV surface antigen was expressed by the hepatocytes in vivo. On successive injections of the HBsAg genes, the antibody to HBV surface polypeptides was produced in the rats, and characteristic pathological changes of lymphocytic focal necrosis and denaturation of hepatic cells were observed in the liver of all the rats. We conclude that hepatitis is caused by the direct injection of HBsAg genes.