Abstract A previous report of a large, double blind, efficacy trial of an experimental Group B meningococcal outer membrane protein vaccine carried out in Norwegian Teenagers, showed a protection rate of 57%. Previous studies had demonstrated the occurrence of mutations in the class-1 outer membrane protein which alter its immunological properties. The occurrence of new mutations might compromise the efficacy of a vaccine and explain the occurrence of any vaccine failures. The porA gene, which encodes expression of the class 1 protein, was sequenced in all isolates from vaccine failures and compared to that of the vaccinating strain H44/76 (B:15:P1.7,16). The porA DNA and deduced amino acid sequences were all identical to that of the vaccinating strain except for that of one isolate which had a sequence identical to strains previously reported in Norway and England with a 'masked P1.7' epitope. The absence of new mutations in the trial was encouraging for the further development of outer membrane protein vaccines.