A total of 571 specimens obtained from the Heidelberg outpatient clinic for sexually transmitted diseases (STD) and 254 specimens sent in by mail were examined for Chlamydia trachomatis, using a direct immunofluorescence technique with monoclonal antibodies and tissue culture. With respect to tissue culture, the sensitivity of specimens from male STD patients was 77% and specificity was 99%; for specimens from female STD patients the corresponding values were 77% and 96%, respectively. In female prostitutes the sensitivity of specimens reached only 55%, and specificity in this group was 96%. Follow-up of the patients with discrepant results confirmed the results of the test, which had initially indicated the chlamydial infection. In the material sent in by mail, the detection rate of C. trachomatis by direct immunofluorescence with monoclonal antibodies was significantly higher than in tissue culture. When specimens have to be transported long distances, the direct test with monoclonal antibodies has proved to be superior to tissue culture. However, the advantage of this test (Micro Trak) is counterbalanced by a relatively low sensitivity.