Chlamydia trachomatis is the major cause of preventable blindness in the world. In areas where trachoma is prevalent, virtually every child becomes exposed and may develop the disease. C. trachomatis is the cause of the most common sexually-transmitted disease (STD) in the U.S. as well as other countries and is a major cause of tubal infertility in women which may go unnoticed in many individuals. Both diseases may be treated with antibiotics but may be either logistically difficult as in the treatment of trachoma, or as in chlamydial STD, they may not be administered at all because the patient is unaware of the presence of the disease. The only truly effective control of both diseases would be prophylaxis with a vaccine. Patients with chlamydial genital tract infections respond with antibody to a variety of chlamydial proteins, in particular, the major outer membrane protein (MOMP) and a 60-62K membrane protein. The incidence of the response to MOMP is variable while most patients respond to the 60-62K protein. To determine which antigens are the best candidates for a vaccine, an animal model will be used. Guinea pigs can be infected in the eye and genital tract with the chamydial agent of guinea pig inclusion conjunctivitis (GPIC), a natural parasite of the guinea pig, which produces diseases analogous to the human infections. Guinea pigs also develop antibodies to MOMP and 61K just as humans. Female guinea pigs will be infected either in the genital tract or eye with GPIC and the antibody response in serum and secretions determined by immunoblot analysis. Cell-mediated immunity (CMI) to whole GPIC, MOMP, and 61K will also be measured by blast transformation. Animals will be challenged at varying times, according to the responses to MOMP and 61K to determine whether either is associated with protection. Animals will also be immunized with whole GPIC, MOMP, and 61K to determine if protective immunity can be induced. Various regimens will be used to stimulate preferentially the mucosal-associated lymphoid system. Immune animals will also be teated with anti-thymocyte serum to determine whether CMI is required for immunity.