Results from the serum bactericidal assay (BA) after immunization of human volunteers with the Norwegian serogroup B meningococcal outer membrane vesicle vaccine are surveyed. In the phase II trials with adults we found very high seroconversion rates (greater than 98%) against the vaccine strain in the BA. Details in the antigenic composition of the inoculum used in the BA seem very important as shown here by finding lower bactericidal titres with teenager sera when tested with a variant of the standard inoculum. The present preliminary report corresponds to the presentation given at the Report Meeting on the Norwegian Meningococcal Vaccine Trial, Oslo, 12 September, 1991.