A simple, direct molecular hybridization test was employed to detect hepatitis B virus (HBV) DNA sequences in human serum. In 61 HBsAg carriers, many with HBV-related diseases (chronic persistent hepatitis, chronic active hepatitis, or posthepatitic cirrhosis), 28/28 (100%) who were HBeAg+ and 16/32 (50%) who were anti-HBe+ had HBV DNA sequences in their serum. Among 22 South African black patients with hepatocellular carcinoma, 7 (32%) had detectable HBV DNA in their serum but at reduced levels when compared to HBsAg carriers without hepatocellular carcinoma, suggesting that viral replication is suppressed or inactive in many hepatocellular carcinoma patients. Hybridization analysis also distinguished carriers with high, moderate, or low amounts of HBV DNA in serum. Ten to 20% of HBsAg+/HBeAg+ carriers showed high serum levels of HBV DNA but, surprisingly, a similar percentage of HBsAg+/anti-HBe+ carriers also showed relatively high serum levels of HBV DNA. Five patients who had undergone immunosuppression therapy and most of whom were on chronic hemodialysis had very high serum levels of HBV DNA, in the range observed during acute HBV infection. By epidemiologic analysis, two of these individuals were implicated in transmission of hepatitis to other hemodialysis patients, paramedical personnel, or intimate family contacts. Serum HBV DNA hybridization analysis identifies carriers with high serum levels of HBV irrespective of HBeAg/anti-HBe status and may define individuals with potentially high risk of transmitting infection to their immediate contacts. enhanced the study of hepatitis B virus (HBV)-related diseases through detection of specific HBV DNA sequences in human tissues. Recently, we and others reported the use of molecular hybridization to detect HBV DNA sequences in liver and hepatocellular carcinoma (1)(2)(3)(4)(5). This technique can also be applied to detect HBV DNA sequences in nucleic acid extracts of serum (6-9). The hybridization method has now been simplified to detect HBV DNA sequences directly in human serum.The objectives of this investigation were to compare immunologic markers of HBV infection with detection of HBV DNA in serum and to determine the relative viral load by direct measurement of HBV DNA sequences. Particular emphasis was placed on HBeAg/antiHBe status in relation to the presence or absence of HBV 285