A common antigenic determinant on the chlamydial major outer membrane protein was detected on each of the three Chlamydia trachomatis biovars (trachoma, lymphogranuloma venereum, and mouse). This determinant was prominently displayed on the surface of chlamydial strains from both the trachoma and lymphogranuloma venereum biovars. However, detection of this determinant on a mouse biovar strain required denaturation by sodium dodecyl sulfate or periodate oxidation. This determinant provides a definable taxonomic link between the three biovars of C. trachomatis.