Chlamydial infection is responsible for a wide spectrum of diseases of the eye, genitourinary tract, and lung. This group of organisms is also implicated in the pathogenesis of coronary artery disease as well as arthritis. Since cross-species infection is widely reported (though probably underestimated), it is an advantage to have a rapid and reliable method to detect all forms of chlamydiae in patient samples. We have identified a 160/163-bp DNA fragment in Chlamydia which is highly conserved in all chlamydial species. A polymerase chain reaction method based on this sequence has been developed to detect, in clinical samples, chlamydiae which have been shown to be positive by fluorescent-staining immunoassay; this method can be utilized in combination with restriction endonuclease cleavage to identify individual chlamydial species. Thus we have developed a sensitive and rapid detection method and have used it on samples from patients with respiratory and genital infections.