In the present study, a monoclonal antibody (mAb), D5-14, raised in our laboratory against Chlamydia trachomatis LGV2 serotype, stained Simkania negevensis inclusions in S. negevensis-infected cells by using the immunofluorescence test. D5-14 mAb, reacting in immunoblot with an approximately 64-66-kDa protein of C. trachomatis LGV2 serotype, recognized a protein with the same molecular mass when tested with S. negevensis elementary bodies.