Growth of Legionella pneumophila serogroup 1 in yeast extract broth was standardized, and protein profiles of detergent-solubilized cells were determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Over 30 protein bands were identified, 6 of which were more prominent both in Coomassie brilliant blue-stained profiles and in fluorograms with intrinsically radiolabeled bacteria. These major proteins were 22,000 dalton (22K), 24K, 43K, 63K, 76K, and 78K. We found that the 24K and 63K major proteins were antigenic, as demonstrated both by radioimmunoprecipitation (RIP) of [35S]methionine-labeled organisms and by immunoblotting with rabbit hyperimmune sera. In addition, both techniques detected antigens migrating at 58K, 72K, 76K, and 78K. The major 22K and 43K major proteins and antigens migrating at 25.5K, 29K, and 46K were only detected by radioimmunoprecipitation, whereas antigens of 19K, 48K, 53K, and 68K were detected only by immunoblotting. Cell-surface localization of the proteins was determined by a modified radioimmunoprecipitation assay designed to react specifically with surface antigens and by the use of hyperimmune sera that had been extensively preabsorbed with intact cells to deplete the sera of antibodies directed against surface components. The 22K, 24K, 43K, 63K, and 78K major proteins and several minor proteins were found to be located on the surface of L. pneumophila cells.