Serum from 221 black patients with hepatocellular carcinoma was tested for HBsAg using a polyclonal radioimmunoassay, and first and second generation monoclonal radioimmunoassays designated M1-RIA and M2-RIA. These monoclonal radioimmunoassays have a lower limit of detection of about 55 and 15 pg of HBsAg-associated epitopes per ml of serum. Correlations were made with other hepatitis B virus serologic markers such as anti-HBc and anti-HBs, using polyclonal radioimmunoassays. We found 47.5% (105/221) of the patients were HBsAg positive by conventional polyclonal radioimmunoassay; all of these patients were also reactive by monoclonal radioimmunoassay. Of the 116 polyclonal radioimmunoassay-negative patients, 4 (3.4%) were reactive by M1-RIA. These four patients were all positive for anti-HBc and/or anti-HBs antibodies. There were 106 of 112 patients negative for HBsAg by polyclonal radioimmunoassay and M1-RIA available for testing by the M2-RIA; 11 (10.4%) were found to be positive only by this test. Thus, with the use of M1- and M2-RIAs, the number of hepatocellular carcinoma patients negative for HBsAg by polyclonal radioimmunoassay was reduced by 14% in this population. More importantly, of the 11 M2-RIA-positive patients, six demonstrated anti-HBc and/or anti-HBs antibodies whereas the remaining five had no serologic evidence of recent or past hepatitis B virus exposure. Finally, of the 21 patients in this series with no markers of hepatitis B virus infection using polyclonal radioimmunoassay and M1-RIA, five (24%) were reactive for HBsAg-associated epitopes by M2-RIA. Taken together, these findings may be due in part to the detection of HBsAg in immune complexes, particularly in those patients with anti-HBc and anti-HBs. However, in hepatocellular carcinoma patients with no hepatitis B virus serologic markers, the M2-RIA may have been reactive due to low levels of antigen expression co-existing with an inadequate or lack of a host immune response to HBcAg and HBsAg, variable expression of hepatitis B virus gene products or to a “hepatitis B-related virus”.