Cross-reactions of 17 members of the family Legionellaceae were studied by four different serological techniques: immunofluorescence (IF), slide agglutination (SA), microagglutination (MA) and immunodiffusion (ID), using antigens and rabbit antisera prepared in our laboratory. Results obtained corresponded closely with those described by other authors, especially for IF and SA. The 17 antigens were further tested by IF with a panel of sera previously diagnosed as positive for legionella. A high number of positive reactions with several of the antigens tested were found, half of them being positive for Legionella pneumophila serogroup 1, usually in combination with other serogroups or species. The remaining sera presented a great variety of patterns combining different antigens.