Satisfactory methods for the serodiagnosis of Chlamydia trachomatis have been widely discussed in recent years. Until a decade ago, the complement-fixation test measuring group-specific antibody was the most widely applied technique. However, despite showing relatively high diagnostic sensitivity in systemic chlamydial infections, it is of little value in the serodiagnosis of localized chlamydial oculo-genital infections or of trachoma. The more recently developed microimmunofluorescence (micro-IF) test is not only a very sensitive technique, but can also reach a high degree of serodiagnostic specificity in certain chlamydial infections. Unlike the complement-fixation test, it measures type-specific antibody which may be directed against one or more C trachomatis serotypes. By nature, chlamydia infections often lead to high background rates of antibody in affected populations. Interpretation of serological results may therefore be difficult, particularly when only single-serum samples are available. However, in seroepidemiological work, in the study of transmission patterns and in the detection of predominant serotypes in a community, the micro-IF test is especially valuable, and if results are interpreted with care, the test may also serve as an indicator of chlamydial infection. The predictability, sensitivity and specificity of serological tests for C. trachomatis will be discussed.