Chlamydia trachomatis is an obligate intracellular bacterium that replicates in a phagosomal inclusion within a eukaryotic host. Light and transmission electron microscopy studies with monoclonal antibody TIB. 131 detected an intermediate filament (IF) epitope, demonstrating that it is present and co-accumulates in the inclusion along with chlamydia-specific antigens. Light microscope studies established that the material constituted a distinctive mass at the inclusion site and remained associated with elementary bodies when cells lysed to release the infectious particles. Transmission electron microscopy demonstrated that material binding the IF-specific monoclonal antibody lies within the inclusion; it is filamentous and entwines maturing reticulate bodies. An enzyme immunoassay, with mature chlamydial elementary bodies isolated and purified from infected embryonated eggs, examined the reactivity of the anti-IF monoclonal antibody; its binding to elementary bodies was readily titrated, indicating that the intermediate filament-positive epitope, or one cross-reactive with it, was present. Thus, the epitope is retained during purification, and its presence is not a function of growth in McCoy cells.