Antigens of the outer membrane of Legionella pneumophila were investigated by means of the immunoblotting-technique using rabbit antisera against three different formaldehyde-inactivated strains, and one heat-inactivated strain of L. pneumophila serogroup 1. Nitrocellulose blots were prepared from membrane fractions extracted with sodium-N-lauryl-sarcosinate from 14 strains of L. pneumophila (eight strains of serogroup 1, and one strain each of serogroups 2-7) and 12 strains of gram-negative rods of various species. After incubation with 125I-protein A or 125I-anti-rabbit IgG immune complexes were identified. These results were compared with Coomassie-stained and silver-stained SDS gels. There was a diffuse reaction in the homologous system between 20 and 80 kilodalton (kDal) after incubation with 125I-protein A, and an intense reaction between 22 and 29 kDal after incubation with 125I-anti-rabbit IgG. Membrane preparations of the different strains of serogroup 1 exhibited clearly discernible patterns. Immunoblots of formaldehyde-inactivated strains when reacted with antiserum against heat-inactivated immunogen showed a single species-specific antigen of approximately 22.5 kDal which could not be assigned to a major protein. Immunoblots of the same antiserum but with heat-inactivated cell wall preparations gave a second species-specific band of approximately 65 kDal. Antisera against formaldehyde-inactivated bacteria demonstrated more complex characteristic patterns, with protein-associated components occurring at 29, 44, 46, 48, 65 and 80 kDal; in addition, cross-reacting fractions were present at 15.5, 17.5 and 22.5 kDal. The 29 kDal major outer membrane protein was immunogenic in most but not all cases.