The sensitivity of the indirect immunofluorescence (IFA) test in Legionella pneumophila infection is said to be maximal when a plyimmunoglobulin conjugate is used. However commercially available non-class-specific fluorescent antisera are not always sensitive enough to detect IgM antibodies as class-specific conjugates do. IFA test's drawback is its inability to detect early stages of infection. We routinely performed the microagglutination (MA) test in order to check the reliability of this test alone in screening diagnostic work for L. pneumophila group 1 infections. The 252 sera tested were from suspected or confirmed legionellosis cases. Five-hundred and thirty sera from healthy-people, 49 sera from patients with serologically confirmed chlamydia, coxiella and mycoplasma pneumonia, and ten sera from patients with Pseudomonas aeruginosa infection were used as controls. There was a good agreement between IFA and MA tests, the MA proving almost as specific as, and in some cases more sensitive than the IFA test. This was particularly evident in early stages of infection. For these reasons, together with its low cost and the ease to perform, it appears that the MA test can be a useful screening test for presumptive cases of legionellosis even on a single serum specimen.