Anti-pre-S2 antibodies were detected by enzyme-linked immuno-absorbant assay using a synthetic peptide analogue of pre-S2 protein, in different groups of hepatitis-B-infected subjects, including patients presenting with cirrhosis and liver cancer, and also in infants immunized with hepatitis B vaccine. Anti-pre-S2 antibodies were not detected in hepatitis B surface antigen (HBsAg) chronic carriers, including patients with cirrhosis or primary liver cancer. Anti-pre-S2 antibodies were not detected in HBsAg-positive sera during the early phase of acute hepatitis. They were only noted upon recovery, when anti-HBs antibodies are detectable at the same time as HBsAg. After recovery, anti-pre-S2 antibodies were noted in 57% of test sera and were still detectable in 16% of anti-HBs-positive sera obtained years after HBV infection. Anti-pre-S2 antibodies were detected in 70% of infants immunized with 2 or 5 micrograms doses of Hevac B Pasteur vaccine, confirming that this vaccine contains pre-S2 antigen. Anti-pre-S2 detection was correlated with the anti-HBs antibody titre.