The prevalence of antibodies to 13 Legionella antigens was compared in three populations: 583 blood donors, 140 tuberculosis patients and 66 patients with acute non-legionellosis pneumonia. Antibody levels were determined by indirect immunofluorescence (IFA) using formalin-fixed antigens prepared from bacteria developed in embryonated hen yolk sac. The very weak prevalence of anti-L. pneumophila antibodies in a healthy population [almost 0 for serogroups (SG) 2, 3, 4 and 5; 1.5% for SG 6 and a maximum of 2.5% for SG 1 at a titer of 1: 16] confirms the criteria that have been recommended by the Centers for Disease control, USA. For the other legionellae studied, these criteria cannot be applied due to a higher prevalence in healthy populations (14.5% with levels of 1: 16-32 and 1% with levels of 1: 64-128 for L. bozemanii and progressively decreasing by as much as 2% for the other species: L. micdadei, L. longbeachae, L. gormanii, L. dumoffii and L. jordanis), in tuberculosis patients (12.1% with a level at 1: 64-128 for L. bozemanii, and for the same titer, 9.2% for L. gormanii, 5.7% for L. micdadei, 5% for L. longbeachae 2), and also in acute pneumonia (2 to 3.3% with levels of 1: 64-128 for L. longbeachae 1 and 2, L. jordanis, L. dumoffii, L. micdadei and L. bozemanii). Although the significance of these prevalences remains to be discussed, with formalin-fixed antigens, it seems reasonable for these species to assign a threshold of 1: 256 for the presumptive serodiagnosis following seroconversion.