Neisseria meningitidis serogroups B and C were originally subdivided into serotypes by the use of two different classification systems based upon type-specific bactericidal antibodies and immunoprecipitation in agar gels, respectively. The serotype specificities were later found to be associated with different major outer membrane proteins and with the lipopolysaccharide. Physiochemical characterization of the four to five major outer membrane proteins has resulted in designation of classes 1-5 on the basis of peptide mapping. The class 2 (41,000-dalton) or class 3 (38,000-dalton) protein is present in all meningococcal strains. They both are predominant proteins, show a useful degree of antigenic variation, and are epidemiologically relevant. A new serotyping nomenclature and scheme based on these two proteins and the lipopolysaccharide is therefore proposed. With use of this scheme, a meningococcal strain could be identified by serogroup to protein-serotype to lipopolysaccharide serotype, as in B:2a:L3. The class 1 (46,000-dalton) protein can be used to further define a meningococcal strain, as in B:2a:P1.2:L3.