Urethral Chlamydia trachomatis infection was diagnosed in 204 of 1,011 (20.2%) male patients by cell culture, in 219 (21.7%) by an antigen detection test consisting of a solid phase immunoassay, and in 247 (24.4%) patients by both methods combined. The positive results of the two methods agreed for 176 patients, and both positive and negative results of the tests agreed for 940 patients (93%). With cell culture as the reference method, the antigen detection test had a sensitivity of 86.3%, a specificity of 94.7%, a positive predictive value of 80.4% and a negative predictive value of 96.5%. It gave false negative results in 28 patients. In 43 patients the antigen detection test gave a positive result, whereas culture was negative. Thirty-nine of these males were treated with antibiotics (tetracycline or erythromycin), 19 because their consorts had a proven Chlamydia trachomatis infection, and 20 for obvious clinical and/or microscopic findings of urethritis requiring treatment. According to this analysis there were 19 probable misses by cell culture test and four true false-positives by the antigen detection test, i.e. less than 0.4% of all patients examined. Since one-third of males with a final diagnosis of Chlamydia trachomatis infection were clinically asymptomatic efforts to control genital chlamydial infections must identify this reservoir. The antigen detection test provides an alternative diagnostic method to the more laborious and time-consuming cell culture procedure.