The present study was performed to identify pathogenic factors of Chlamydia trachomatis, which invade the host cell membrane. We prepared monoclonal antibody against C. trachomatis and searched for pathogenic factors using this antibody, and subsequently identified the surface components of the elementary body of C. trachomatis, i.e., major outer membrane protein (MOMP), lipopolysaccharide (LPS), and two other surface exposure proteins. These proteins are believed to be important in the pathogenesis of host cell chlamydial infection. Additionally, to identify factors related to the host cell and C. trachomatis, we prepared C. trachomatis infected and non-infected McCoy cell extracts, and reacted these with anti-chlamydial LPS monoclonal antibody. We found that anti-chlamydial LPS monoclonal antibody reacted with a 116 kDa proteinaceous McCoy cell membrane component.