The increasing worldwide recognition of the deleterious sequelae of Chlamydia trachomatis urogenital tract infections makes rapid, inexpensive diagnostic tests a high priority. Cell culture, still the standard for all other methods, is only 90% sensitive. Direct fluorescent antibody (FA) tests use highly specific monoclonal fluorescing antibodies directed against the elementary body, permitting direct examination of urethral and endo-cervical smears. These tests are highly sensitive (92% to 95%) and specific (95% to 96%). Major drawbacks are cost, need for a fluorescence microscope, the care required to collect and prepare adequate smears (up to 10% of smears are unacceptable), and the expertise required to interpret slides. By means of enzyme-linked immunoassay (EIA) tests large batches of specimens can be tested in a few hours; they require little technician time and no subjective interpretation, but their sensitivity appears to be less than that of the direct FA tests. Analysis of Gram-stained urethral and endocervical specimens, combined with gonococcal cultures in women, is still the cheapest, most rapid method of establishing a presumptive diagnosis of chlamydial urethritis and endocervicitis. In sexually transmitted disease clinics it is still the method of choice. An EIA test can be used for high-volume screening of women but should be used only in high-risk populations because its predictive value falls sharply as the prevalence of infection decreases.