A bactericidal assay is described which allows identification of distinct serotypes within a serogroup of Neisseria meningitidis. Antisera produced in rabbits against seven group C strains by two intravenous inoculations of live organisms were found to contain two types of bactericidal antibodies. One, directed against the group-specific polysaccharide, caused various degrees of killing of all strains. Absorption of this antibody by purified group C polysaccharide revealed the presence of the second bactericidal antibody. This antibody was directed against antigenically distinct factors associated with serotype specificity. Extensive cross-absorption yielded antisera with activity directed against four separate factors. The presence of a factor in a strain was indicated by its susceptibility to killing by antisera containing antibody to that factor. A serotype was defined by the particular combination of factors. Six different serotypes, containing one or two factors, were identified among 16 group C strains examined.