The method of hemagglutination inhibition was used to investigate the antigenic diversity of lipopolysaccharide (LPS) from Neisseria meningitidis and to develop a serotyping systems based on this antigen. The system uses outer membrane complex prepared by a simple extraction procedure to inhibit homologous hemagglutination reactions involving sheep erythrocytes sensitized with purified LPS and rabbit antiserum raised to whole meningococci. Antisera with specificity for eight different LPS determinants were used as typing sera to serotype a cross section of 67 meningococcal strains. Only two strains (both group A) were not typable with the eight sera, and most strains had more than one type. Comparison of LPS type and bactericidal serotype suggests that the LPS and protein serotypes are independent serological markers.