Recently, additional polypeptide components of the surface envelope of hepatitis B virus (HBV) have been identified. The pre-S(1) and pre-S(2) regions of the HBV genome encode NH2-terminal amino acid residues that together with the S-gene product (25 kDa) comprise polypeptides of 33 kDa and 39 kDa. The possible immunopathologic significance of these larger polypeptides and their relevance to vaccine development prompted us to examine the murine immune response to pre-S(2)-encoded determinants as compared to S-encoded determinants on the same polypeptide. Previous work showed that the pre-S(2) region elicits greater antibody production in vivo than does the S region of hepatitis B surface antigen. In this study, we examined immunogenicity of the pre-S(2) region at the T-cell level, H-2- and non-H-2-linked genetic influences on the pre-S(2) response, and the effect of the immune response to one region on the immune response to the other region. The results indicate that (i) the pre-S(2) region is significantly more immunogenic than the S region at the T-cell level; (ii) pre-S(2)-region-specific T-cell activation is regulated by H-2-linked genes and correlates with the H-2 restriction of in vivo antibody production to the pre-S(2) region; (iii) the H-2 restriction of the T-cell response to the pre-S(2) region is distinct from the H-2 restriction of the T-cell response to S-region determinants; (iv) non-H-2-linked and non-Igh-linked genes also influence the humoral immune response to the pre-S(2) region; and (v) immunization of an S-region-nonresponder, pre-S(2)-region T-cell-responder strain with HBV envelope particles containing both the pre-S(2) and S regions can circumvent nonresponsiveness to the S region through pre-S(2)-specific T-cell helper function.