Antibodies to the class 2/3 OMPs (PorB) are bactericidal and antigenic diversity between these proteins forms the basis of the current serotyping classification. We have examined the variable regions of the porB gene from the 13 serotype strains expressing class 3 OMP, and have developed a panel of oligonucleotide probes for identification and classification of strains using genetic techniques. We have used this system of molecular analysis in several ongoing projects: Molecular epidemiology of meningococcal meningitis in Brazil using PCR amplification of the porB gene directly from cerebrospinal fluid and subsequent genotyping using oligonucleotide probe hybridization. Examination of serotype 15 strains obtained from cases of meningococcal disease among vaccine recipients and controls from the Norwegian outer membrane protein vaccine trial. Investigation of an unusual disease isolate from Chili, in collaboration with the CDC, which binds both type 4 and type 15 monoclonal antibodies. Using our probe hybridization system, we identified one of the four porB variable regions (VR1) that was identical to the VR1 of type 4 strains and three variable regions (VR2, VR3, VR4) that were identical to those of type 15 strains. Rabbit immunization studies are suggest immunodominance of single individual epitopes corresponding to specific variable regions.