SUMMARY.The product of the pre‐S plus S gene of hepatitis B virus appears to be more immunogenic in mice than the S‐gene product (HBsAg) alone. Therefore, we tested the immunogenicity in healthy adults of a hepatitis B vaccine containing the ‘middle protein’ gene product of pre‐S2 plus S (pre‐S vaccine). We compared the immunogenicity of three doses of the pre‐S vaccine with that of a commercially available recombinant hepatitis B vaccine (Recombivax‐HB®): 87 seronegative adults were randomized to receive 12 μg (group 1), 24 μg (group 2), or 48 μg (group 3) of the pre‐S vaccine or 10μg of Recombivax‐HB (group 4) by deltoid injection at 0, 1 and 6 months. Antibody to HBsAg (anti‐HBs) appeared after booster vaccination in 94% of vaccinees. Immunogenicity was best in recipients of 48 μg of the pre‐S vaccine and Recombivax‐HB, and geometric mean titres (GMT) for the pre‐S vaccine were higher than those for Recombivax‐HB only at the pre‐S vaccine dose of 48 μg (group 3). Antibody to pre‐S2 developed in 75% of the pre‐S2 vaccine recipients (not in Recombivax‐HB recipients) within 7 months. These findings indicate that the pre‐S vaccine is immunogenic in healthy adults but that a dose of 48 μg of the current formulation is required to equal or exceed the immunogenicity of currently available, recombinant S‐only vaccine. Studies in non‐responders to S‐only vaccines will be necessary to define an immunological advantage of the pre‐S vaccines, and additional assessments will be necessary to det