The major outer membrane protein (MOMP) of several Chlamydia trachomatis serotypes was isolated by preparatory SDS-PAGE and used to raise hyperimmune rabbit antisera. The specificity of these antisera is being analyzed by: (i) rocket immunoelectrophoresis and immuno-blotting of SDS-denatured MOMP, (ii) indirect immunofluorescence of purified elementary bodies (EBs), and (iii) in vitro neutralization of chlamydial infectivity. By immuno-blotting analysis with anti-MOMP sera, all C. trachomatis MOMPs showed strong immunological cross reactivity demonstrating that SDS-insensitive species-specific antigens are the primary determiners measured by this technique. In contrast, these same antisera showed considerable serotype specificity when tested by both immunofluorescence staining of EBs and neutralization of infectivity. These results show that the chlamydial MOMP is a complex macromolecule antigenically. Furthermore, they indicate that the antigenic unique portions of the protein are more exposed on the organisms' surface, whereas, the antigenically common portions, demonstrated by immunoblotting, may be located within the bilayer of the outer membrane.