Sets of sera (444) submitted for diagnostic testing for legionellosis were tested against 29 indirect immunofluorescence assay (IFA) antigens prepared from the characterized Legionella species and Legionella-like organisms to determine the prevalence of antibodies to Legionella organisms. Reciprocal titers of 15% of the serum sets rose fourfold or more to greater than or equal to 128 (indicating seroconversion) against one or more Legionella antigens. The specificity of the test was 96% when evaluated in patients with pneumonia due to non-Legionella organisms. Antibodies were of the IgG, IgM, and (infrequently) IgA classes and were either specific for a single species (as defined by a difference in titer of fourfold or more) or reacted with common Legionella antigens (30 [45%] vs. 36 [55%] of 66 seroconversions, respectively). No single antigen detected half of the positive sera. Elevated IFA titers (of greater than or equal to 256) against single or multiple Legionella antigens occurred in 12% of 184 normal control sera. Therefore, only seroconversions to titers of greater than or equal to 128 should be considered indicative of recent Legionella infection.