Three different formulations of an outer membrane vesicle (OMV) vaccine against group B meningococcal disease have been prepared and tested for immunogenicity and reactogenicity in adult volunteers. The vaccines were prepared with or without aluminium hydroxide and serogroup C-polysaccharide (C-ps). Doses from 12.5 to 100 micrograms protein were given twice at a six weeks' interval. All three formulations were well tolerated and highly immunogenic, inducing bactericidal and opsonizing antibodies in humans. Adsorption of OMVs to aluminium hydroxide reduced the pyrogenicity in rabbits. The differences in immunogenicity between the formulations were relatively small, but after the second dose a stronger booster response was observed when the vaccines were adsorbed. Thus, a formulation with OMVs and C-ps represents a safe and highly immunogenic vaccine, even without aluminium hydroxide.