The protein composition of the outer membranes of eight serogroups of Legionella pneumophila has been determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Outer membranes were prepared by detergent extraction using sodium lauryl sarcosinate or by isopycnic sucrose gradient centrifugation. With both techniques one major outer membrane protein of about 29,000 daltons was found to be characteristic for the species L. pneumophila. It was the predominating feature in all 22 strains of L. pneumophila studied, regardless of serogroup. SDS-PAGE patterns of non inactivated L. pneumophila strains were compared with those following formaldehyde-, heat- or ether inactivation. Formaldehyde inactivation gave the fewest protein bands while the outer membrane protein profiles of non inactivated as well as of heat- or ether-inactivated strains revealed some additional minor components. With the exception of a 46,000 dalton band that showed, in some strains, an altered electrophoretic mobility of ca. 48,000 dalton, all strains and serogroups of L. pneumophila presented with the same outer membrane protein pattern. Analysis of outer membrane protein profiles by SDS-PAGE should therefore be a valuable tool for the identification of L. pneumophila. Comparing total membrane preparations the 29,000 dalton component was also the predominant feature, an appreciable number of additional bands, however, allow a clear discrimination between different strains. The protein profiles of outer and total membranes of L. pneumophila as determined by SDS-PAGE therefore may be used for taxonomical and epidemiological studies.