A study was undertaken to establish the background prevalence of antibodies to L. pneumophila in 324 persons from the area of Seville, Spain. The serum specimens were obtained from healthy volunteers divided into groups with different risk factors. The results were as follows: a prevalence of 1.96% in 51 persons working in a cooling-tower air conditioned building (group A), a prevalence of 25% in 36 persons working in a dry-heat air conditioned building (group B), a prevalence of 16% in 87 underground construction workers (group C) and a prevalence of 1.2% in 150 healthy blood donors with none of the above risk factors (group D). There is statistical significance between D and B and C.