The appearance of Legionella pneumophila antigen in the urine of guinea-pigs with experimental airborne legionnaires' disease was investigated and compared with that of emerging antibody by use of enzyme-linked immunosorbent assay. Models with high dose (acute) and low dose (chronic) infection were studied. Antigen was detected after 40 h in the high dose group but animals died (at 3 days) before an antibody response could be elicited. In the low dose group, antigen was detected 4 days after infection, well before serum antibody was detected (7-10 days). Small, but significant, amounts of antigen were detected up to 17 days after infection in surviving animals. Although detection of L. pneumophila antigen in urine has been proposed before, and achieved on an ad hoc basis, the technique is not in routine, general use. This is due mainly to difficulties of evaluation in relation to other methods of early diagnosis in the human situation where infectious dose, time of infection, host uniformity and availability of samples present difficulties. The use, in this study, of a highly relevant aerosol-infected guinea-pig model of legionnaires' disease has avoided these uncertainties and hopefully proved the value of this technique for general routine use. The antigen detection test was shown to be rapid, sensitive and reliable, and allowed diagnosis of legionnaires' disease earlier than was possible by demonstrating antibody. In addition, the detection of antigen in urine is a convenient and non-invasive procedure.