Variation in the protein and lipopolysaccharide composition of the meningococcal outer membrane may be due to either serotype differences or to changes in cultural conditions. There are 12 antigenically distinct serotypes of group B meningococci, and these are associated with distinct major outer membrane protein patterns on sodium dodecyl sulfate-polyacrylamide gels. In most strains the predominant outer membrane protein carries the serotype-specific determinant. Certain strains, when grown under similar conditions in different media showed an altered membrane composition. The type 2 strain, M986, grown in modified Frantz medium-A, had a reduced amount of the major 41,000-dalton protein while a 28,000-dalton protein predominated. The altered protein composition may be related to changes in cell metabolism as reflected by the pH of the medium after growth. Growth of the organism in Frantz medium-B caused a negligible drop in pH and the 41,000-dalton protein remained predominant. There was also variation associated with changes in the growth rate. Increasing the aeration caused a concomitant increase in growth rate and cell yield. We observed two quantitative changes in outer membrane proteins in four of seven strains examined: (i) where only a single major protein changed (three strains), and (ii) where an increase in one protein component was associated with a decrease in another protein (one strain). When the strains were grown in tryptic soy broth (Difco Laboratories, Detroit, Mich.) with either high or low aeration, the total protein in the outer membrane remained constant. In contrast, with high aeration there was a significant increase in lipopolysaccharide. These studies suggest that the cell surface proteins may be altered by the organism to meet a variety of environmental conditions.