Chlamydia trachomatis isolates from 58 patients were immunotyped by radioimmunoassay (RIA) and fluorescent-antibody staining (FA) by using monoclonal antibodies. Unambiguous typing was observed with 55 isolates by FA and 53 isolates by RIA, although 3 isolates were typed by RIA but not FA. FA required less antigen and was more sensitive in detecting mixed infections.