Based on differences in reaction pattern with monoclonal antibodies against the P1.16 epitope, a new variant of the class 1 protein in Neisseria meningitidis serogroup B was identified in Norway. A single amino acid deletion was revealed when the part of the gene region encoding the second variable region of the protein was sequenced. This new variant was designated P1.16c. About 5% of the B:15:P1.7,16 strains in Norway from the time period 1987-1991 were P1.16c. In a localized area in Southern Norway, 5/8 (62%) of the P1.7,16 strains were P1.16c. The P1.16b mutant, recently described in England, was not found among the Norwegian meningococcal isolates. Strains carrying the P1.16c mutation showed increased resistance to bactericidal killing, not only by P1.16-specific monoclonal antibodies, but also by the sera from individuals immunized with a vaccine based on outer membranes from a B:15:P1.7,16 strain.