Urogenital swabs (571) were investigated with a solid-phase enzyme immunoassay for the detection of Chlamydia trachomatis antigen (Chlamydiazyme, Abbott). The results were compared with the conventional cell culture method (McCoy cell culture). Urogenital C. trachomatis infections were diagnosed with the cell culture in 14 of 122 male STD patients (12%), in 12 of 79 female STD patients (15%), in 23 of 89 prostitutes (26%), and in 3 of 115 asymptomatic males (3%). In comparison with cell culture, the sensitivity of Chlamydiazyme for urethral specimens from male STD patients was 86%. In female STD patients, for urethral specimens a sensitivity of 83% was found and for cervical specimens a sensitivity of 80%. The corresponding values for specimens from prostitutes were 60% and 100%, respectively. The specificity of Chlamydiazyme for urethral specimens of male STD patients reached 95%. With respect to urethral and cervical specimens of female STD patients, the specificity was 88% and 82%, respectively, and in prostitutes 92% each. The low specificity in female patients cannot be ascribed only to Chlamydiazyme since, after subcultivation and detection of inclusions by the use of fluorescein-labeled monoclonal antibodies, some of the false-positive Chlamydiazyme results turned out culture positive. This means that the specificity of Chlamydiazyme is actually higher. Because it can be performed rapidly and simply and reaches detection rates approaching those of the cell culture method, the enzyme immunoassay is an improvement in the diagnosis of C. trachomatis infections.