The cell surfaces of two Chlamydia trachomatis serovars were explored by immune electron microscopy with monoclonal antibodies that recognize a number of chlamydial outer-membrane components. Species, subspecies and serovar-reactive epitopes on the major outer-membrane protein (MOMP) of a lymphogranuloma venereum biovar strain, L2/434/Bu, and a trachoma biovar strain, F/UW-6/Cx, were exposed on the surfaces of both elementary bodies (EBs) and reticulate bodies (RBs). Three epitopes on MOMP were inaccessible on EBs and RBs of both strains. These included a genus-reactive, species-reactive, and a subspecies-reactive epitope. In contrast, genus-specific epitopes on lipopolysaccharide (LPS) were not detected on the EB surface, but were clearly expressed on RBs of both L2/434/Bu and F/UW-6/Cx chlamydiae. Antibodies specific for the 60 kDa and 12 kDa 'cysteine-rich' outer-membrane proteins did not react with surface epitopes on either EBs or RBs. These data provide evidence that MOMP is a major surface antigen of both morphological forms, whereas some portions of the LPS molecule are exposed on the RB surface but become inaccessible to antibody after conversion to the infectious EB form.