A rapid immunochromatographic assay for the detection of Legionella pneumophila serogroup 1 antigen in the urine of patients with pneumonia. It is intended, in conjunction with culture and other methods, to aid in the presumptive diagnosis of Legionella infection causes by L.pneumophila serogroup 1. L. pneumophila serogroup 1 antigen has been detected in urine as early as three days after the onset of symptoms.