The expression of the preSl antigen of hepatitis B virus in sera from chronic HBsAg carriers was studied using a specific monoclonal antibody F35.25 in an original, double-immunoradiometric assay. The antibody F35.25 recognized an epitope located between amino-acid residues 32 and 53 on the preSl sequence of the large HBsAg protein. This domain could be involved in the recognition of hepatitis B virus by hepatocyte receptors. PreS 1 antigen detection by monoclonal antibody F36.25 closely correlated with the presence of complete virions in the serum of HBsAg carriers, as demonstrated by ultracentrifugationgradient experiments and electron-microscopical examination. Of the 19 HBsAg carriers with chronic liver disease, preSl antigen was detected in 17 (90%): all of the 11 HBeAg-and hepatitis B virus-DNApositive cases (group 1) and six of eight anti-HBepositive cases with low levels of hepatitis B virus replication (group 2). PreSl antigen/HBsAg ratios parallel to preS1 antigen titers were significantly higher in the HBeAg-positive group (34% and 1:10*) than in the anti-HBe-positive group (18% and l:lOa). In contrast, preSl antigen was not detected in 18 (90%) of the 20 HBsAg healthy carriers positive for anti-HBe and negative for serum hepatitis B virus-DNA (group 3 ) . Our results show that in chronic HBsAg carriers the serum expression of preSl antigen correlates well with the level of hepatitis B virus replication (serum hepatitis B virus-DNA and/or liver HBcAg) and that it may be useful in assessing the clinical importance of the chronic viral infection. (HEPATOLOGY 1990;11:809-814.) The three envelope antigens (HBsAg) of HBV are now well characterized (1). The major HBsAg protein is encoded by the S gene and consists of unglycosylated (P24) and glycosylated (GP27) forms. The middle HBsAg protein is encoded by the preS2 region and the S gene and consists of two glycosylated forms