Seventeen monoclonal antibodies to one of three main cell surface antigens of Neisseria meningitidis group B were tested for protective efficacy in the infant rat using as challenge seven strains of different class 2/3 protein serotypes, class 1 protein (P1) subtypes and LPS immunotypes. Type-specific protection indicated both by a reduction of bacteraemia and meningitis and survival of the animals was regularly obtained with antibodies to the P1 protein and to LPS. By contrast, only one of seven antibodies to the serotype-specific class 2/3 protein was protective, even though four of them were highly bactericidal. The animal protection test and in vitro bactericidal assay were otherwise concordant. These data form important guidelines for the design of vaccines to prevent group B meningococcal infections.