Lipopolysaccharide was extracted with cold phenol water from Legionella pneumophila and used as antigen for ELISA. IgG and IgM antibodies were measured with the ELISA and the immunofluorescence assay (IFA). Agglutinating antibodies were measured by the microagglutination (MA) test. In tests on sera from 27 patients with confirmed Legionella infections predominantly due to L. pneumophila serogroup 1 the results with the ELISA, the IFA and the MA were compared to each other. Antibody titers obtained by the ELISA were in general much higher than those obtained by both other tests. The ELISA proved to be the most sensitive method (IgG: 91.3%, IgM: 52.2%) whereas the sensitivities of IFA and MA were IgG: 69.6%, IgM: 30.4% and 60.9%, respectively. There was low correlation of the IgG antibody titers but good correlation of the IgM titers. Further 49 sera from patients without Legionella infection were screened to calculate the specificities of the three tests which were equally good with all methods (98%).