An indirect fluorescent antibody (IFA) test was used to establish the background prevalence of antibody to Legionella pneumophila in single serum specimens from 1, 143 persons. The serum specimens had been obtained from volunteers 46 years of age and older who were not acutely ill and who resided in the areas of Atlanta, Georgia; Washington, D. C.; Houston, Texas; and Rochester, New York. The overall prevalence of seroposi-tivity (reciprocal titer, ≥64) was 1.7%. The prevalence of seropositivity did not vary with age, sex, or geographic location. Groups of persons in which the prevalence of reciprocal titers of ≥64 is significantly higher than 1.7% may have unusually great exposure to L. pneumophila. In the population tested, a reciprocal IFA titer of ≥ 64 would have a specificity of 98.3% in the diagnosis of an acute illness as Legionnaires’ disease.