PROBLEM: Chlamydia trachomatis is a significant worldwide health problem, and the often-asymptomatic disease can result in infertility. To develop a successful vaccine, a complete understanding of the immune response to chlamydial infection and development of genital tract pathology is required. METHOD OF STUDY: We utilized the murine genital model of chlamydial infection. Mice were immunized with chlamydial major outer membrane protein, and vaginal lavage was assessed for the presence of neutralizing antibodies. These samples were then pre-incubated with Chlamydia muridarum and administered to the vaginal vaults of immune-competent female BALB/c mice to determine the effect on infection. RESULTS: The administration of C. muridarum in conjunction with neutralizing antibodies reduced the numbers of mice infected, but a surprising finding was that this accelerated the development of severe oviduct pathology. CONCLUSION: Antibodies play an under-recognized role in chlamydial infection and pathology development, which possibly involves interaction with Th1 immunity.