Immune rabbit sera and sera of patients with legionellosis, leptospirosis, Q fever and antibodies against Borrelia burgdorferi were examined in cross reactions. In currently used serological tests, indirect immunofluorescence and the microagglutination test no antibodies were detected which could cause difficulties in the interpretation of results in the diagnosis of legionellosis. In one patient a mixed infection with Q fever and legionellosis was found.

        Key words: serological diagnosis of legionellosis – specificity of tests.