Guinea pigs were infected intraperitoneally with 4 subgroup reference strains (Knoxville 1, Philadelphia 1, Bellingham 1, OLDA) and 2 clinical isolates of Legionella pneumophila serogroup 1. Antigenuria was demonstrated by the enzyme-linked immunosorbent assay using polyclonal antibodies and monoclonal antibody (mab) F8/5. Mab F8/5 recognizes a hitherto undetected common epitope on urinary antigen of the investigated strains.