Two antigens were isolated from each of three strains of the Legionnaires' disease (LD) bacterium. One antigen was serotype-specific; the other cross-reacted with strains of LD bacteria of different serotypes. The serotypic antigens contained all the major branched-chain fatty acids characteristic of LD bacteria and were a lipid-protein-carbohydrate complex. Electrophoresis resolved the serotypic antigen of Knoxville 1 strain into four protein bands and one glycoprotein band with molecular weights ranging from 0.5 to 7.2 x 103. The specificity of the serotypic antigens was established by immunofluorescent staining, and double gel diffusion confirmed the absence of cross-reactions between the serotypic antigens of different types and the partial identity of the serotypic antigens from serologically related strains. The cross-reacting antigens formed precipitin bands with all the homologous and heterologous sera tested. Induction of immunity by vaccination with serotypic antigens was shown in three animal models using guinea pigs and mice. Passive-transfer of immunity (IgG) was also shown in mice and guinea pigs. These observations raise the possibility of a vaccine for protection against infection of LD bacteria.