On the basis of theoretical considerations, a peptide (H Discovery of hepatitis virus-specific antigens (1-3) and their association with the outer membrane ofhepatitis B virus (HBV) (4, 5) has led to rapid advances in the prevention of HBV infection. This has been achieved through detection of HBV carrier blood donors by testing for the HBV surface antigen (HBsAg) (6, 7), and by passive (8-12) and active (13-18) immunization.The protective effect of hepatitis B immune globulin, prepared from plasma selected for high anti-HBsAg content, as well as the demonstration that protective immunity can be induced, albeit at high cost, by the purified 23,000-dalton HBsAg polypeptide isolated from HBV-associated subviral particles (19), has suggested that anti-HBsAg is a protective antibody. Other surface specificities on the HBV virion could also play a similar role (18,20).HBsAg also exhibits generally mutually exclusive strain-specific subtype antigenic determinants-e.g., d/y (21) and w/r (22)-in addition to the determinant a common to all strains. Available evidence suggests that the subtype specificities play no role in providing protective immunity (23,24), and thus that HBsAg/a may be the critical determinant.It is thus ofobvious interest to learn the amino acid sequence of the antibody-combining site, or epitope, of HBsAg/a. Progress towards this goal has depended on two key advances: isolation and characterization of HBV DNA (25) and determination of its sequence through recombinant DNA methods (26)(27)(28)(29). This permitted prediction of the amino acid sequence of the product of the HBsAg gene (the s gene) (30,31). Using a theoretical model, Hopp and Woods (32) were then able to predict a putative dominant HBsAg epitope (termed H epitope): the sequence Lys-Pro-Thr-Asp-Gly-Asn corresponding to positions 141-146 on the HBsAg protein. They then synthesized a tetradecapeptide (H peptide) containing residues 138-149 of HBsAg plus two glycine residues at its COOH terminus, using classical solid-phase peptide synthetic methodology. The four cysteinyl residues were replaced by a-aminobutyric acid in order to prevent polymerization and other side reactions common to sulfhydryl-containing peptides. Polystyrene beads covalently coated with H peptide bound more "2I-labeled anti-HBsAg than did uncoated beads (33).We report herein confirmation of the prediction that the H peptide contains a major epitope for HBsAg, and we further demonstrate that this sequence of amino acids contains the HBsAg/a and HBsAg/d epitopes, but not the epitope ofHBsAg/ y or human serum albumin. Furthermore, we demonstrate that when bound to a carrier the peptide induces the synthesis of anti-HBsAg in vivo.MATERIALS AND METHODS H Epitope Polypeptide Preparations. Initial experiments were done with intact polystyrene beads (XE305A resin beads, Rohm and Hass, Philadelphia) on which the H peptide had been synthesized. These beads contained 10 mg of peptide per 25 mg.For immunization it was necessary to remove the peptide from the polystyrene ...