Sera from 106 southern African blacks with hepatocellular carcinoma and hepatitis B surface antigenemia (HBsAg) were tested for hepatitis B viral DNA (HBV-DNA) activity, HBV-DNA polymerase concentrations, and HBV e antigen (HBeAg) and antibody (anti-HBe) to investigate the state of viral replication in these patients. HBeAg and anti-HBe were detected by radioimmunoassay, HBV-DNA by molecular hybridization using a 32p-labeled HBV-DNA probe, and HBV-DNA polymerase was measured by incorporation of 3H-labeled thymidine triphosphate into double-stranded HBV-DNA. HBeAg was present in 30.2% (32/106) of the patients, almost always in low titer; 63.8% of the patients were anti-HBe positive. Circulating HBV-DNA was detected in 18.8% (20/106) of patients, including 14 of 32 (43.7%) who were HBeAg positive and 6 of 74 (8.1%) who were anti-HBe positive. In most patients, only trace amounts of HBV-DNA were evident. Raised HBV-DNA polymerase activity was found in 5.6% (6/106) of the patients, all of whom were HBeAg positive and 4 of whom had detectable amounts of circulating HBV-DNA. The HBV-DNA polymerase activity was relatively low in these patients. HBV replication thus appears to be present in only a minority of southern African Blacks with HBV-related hepatocellular carcinoma, and when present is of low grade activity.