One hundred and seventy-five human sera were tested for the presence of type-specific antibodies against L pneumophila serogroups 1 to 6 and the Leiden-1 strain by means of an enzyme linked immunosorbent assay (ELISA) and compared with the results obtained by the indirect immunofluorescence assay (IFA). A high correlation (correlation coefficient 0.92) between both methods was found. No consistent pattern of IgG, IgA and IgM classes of antibody titres against L pneumophila were found. In the sera of 15 of 17 patients with a proven L pneumophila pneumonia, IgM class antibodies against L pneumophila could be detected. A "polyvalent" ELISA was developed which permits rapid routine screening of human sera for antibodies against L pneumophila serogroups 1 to 6 and the Leiden-1 strain.