We tested the efficiency of induction of immune responses to the small hepatitis B surface antigen (HBsAg) in mice by intramuscular DNA immunization using different vector constructs that allow high levels of HBsAg expression in mouse cells. The HBsAg-specific responses of class I-restricted cytotoxic T lymphocytes (CTL) and of B cells (serum antibody titers) were measured. Following the intramuscular inoculation of 'naked' DNA, five different vector constructs of 4-8 kb, that contained or did not contain an intron and/or the neo gene, in which HBsAg expression was driven by promoter sequences derived from the immediate early region of HCMV, the SV40 enhancer/promoter region, or a retroviral 3' LTR efficiently primed responses of class I-restricted CD8+ CTL precursors. In contrast, the constructs in which HBsAg expression was driven by HCMV-derived promoter sequences stimulated significantly higher levels of HBsAg-specific serum antibody titers after intramuscular DNA injection than the SV40 or MPSV vector constructs. Large (15 kb) episomal vector constructs did not stimulate CTL or antibody responses. The data demonstrate that: (i) intramuscular DNA immunization represents an efficient technique for priming CTL and antibody responses to HBsAg; (ii) many vectors can be constructed that express an immunogenic product after intramuscular inoculation of 'naked' DNA; (iii) the efficiency of the tested vector constructs to prime after DNA immunization, either a CTL response, or an antibody response, differs.