The immune response to individual structural polypeptides of Chlamydia trachomatis was studied in 75 sera from symptomatic and asymptomatic women with culture-proved genital infections and from apparently healthy women who were culture-negative for C. trachomatis. The immunoblotting technique and the single serovar (L2) inclusion immunoperoxidase assays were used for measurement of the various antibodies. Antibodies to 18 structural polypeptides, ranging in molecular weight from 29 to 204 Kdaltons, were detected by the immunoblotting technique in sera from seropositive women. The immunoperoxidase assay showed that sera with high titers of IgG and IgA antibodies to C. trachomatis reacted with more polypeptides than did sera with low titers in this test. Antibodies to the 60- and 62-Kdalton polypeptides were detected in almost all sera positive for IgG and IgA antibodies, irrespective of chlamydial shedding. About 40% of sera with high IgG and IgA titers reacted with 39-, 57-, 64-, 72-, 86-, 105-, 155-, and 204-Kdalton polypeptides. The prevalence of IgA antibodies to C. trachomatis was higher among women with culture-proved chlamydial infections than among apparently healthy controls.