Urine from nine patients with serogroup 1 Legionnaires' disease, 100 with infected or contaminated urine, 50 with bacteremic infections, 26 with infectious or infiltrative pulmonary diseases, and 65 with miscellaneous infectious or noninfectious illnesses, was examined by radioimmunoassay for the presence of Legionella pneumophila antigen(s). Urine of all nine Legionnaires' disease patients was clearly differentiated from that of the 241 control subjects by the assay. The antigen was detected from 2 to at least 10 days after the initiation of erythromycin therapy. A probable false-positive results occurred in a patient with necrotizing nosocomial pneumonia who died before definitive serologic evaluation for Legionnaires' disease could be completed. The antigen was stable at 100 degrees C for 30 minutes and was not degraded by trypsin. Its elution behavior suggested that its molecular weight is approximately 10 000. The radioimmunoassay appears to be a useful method for rapidly diagnosing serogroup 1 Legionnaires' disease.