Abstract: Procedures are presented for isolating the major outer membrane protein of Chlamydia trachomatis. The isolated protein is a species specific antigen which comprises about 60% of the C. trachomatis cell outer membrane structure. The protein has a molecular weight ranging from about 38,000 to 44,000 daltons, with a mean molecular weight of about 39,500 daltons. The protein antigen is purified from C. trachomatis cells by first extracting the cell contents with a mild anionic detergent, preferably sarcosyl, to leave a residue of intact outer cell membranes. These outer cell membranes are then extracted with a strong anionic detergent, preferably sodium dodecyl sulfate, which solubilizes the 39,500 dalton antigen. The antigen is then purified by hydroxlapatite chromatography. The antigen is species specific for Chlamydia trachomatis and may be utilized in assaying Chlamydial infection in mammals.