Serum hepatitis B antigen-antibody system was detected by ELISA method in 150 patients suffering from hepatocellular carcinoma (HCC) with positive AFP. The overall infection rate of hepatitis B virus (HBV) was 86.0% (129/150). According to the antigens and antibodies to HBV, two major types of seven models could be demonstrated. In type I (including models 1 to 4) HBV infection was present in 81.9%, 74.6% among whom were considered to be of weak infectivity and 7.3% of strong one. In type II (including models 5 to 7) 18.0% (27/150) had no antigen and antibody to HBV was detected, indicating the absence of HBV infection. This study suggests that chronic infection with HBV is the major etiologic agent for human HCC and the patients with HCC often accompanied by active HBV replication.