Adjuvant-active murabutide (N-acetylmuramyl-L-alanyl-D-glutamine-alpha-n-butyl ester), devoid of the side effects of muramyl dipeptide (N-acetylmuramyl-L-alanyl-D-isoglutamine), was administered in saline with natural and synthetic hepatitis B surface antigens (HBsAgs). This derivative enhanced the antibody responses against natural HBsAg. The persistence of high antibody titers was the same in the murabutide-treated group as in the alum-treated group. A synergistic enhancement of the antibody was obtained when both adjuvants were administered together. Cell-mediated immunity to HBsAg, tested by the proliferative response of lymph node cells to the antigen, was also shown to be induced in mice treated with alum-associated murabutide. When administered with natural HBsAg, murabutide produced titers of total antibodies as high as did alum but lower titers of specific immunoglobulin E. High levels of antipeptide antibodies were obtained when a synthetic fragment [HBsAg(99-121)] conjugated to a toxoid carrier was administered with murabutide in saline.