The chlamydial serum immunoglobulin G antibodies of 128 infertile women were determined by a single-antigen (L2) immunofluorescence test. The antibody titers were compared with those of female contacts of men with nonspecific urethritis and with those of "normal" pregnant women. Among infertile women, 43% lacked chlamydial antibodies (titer less than or equal to 8). The corresponding proportion for contacts of men with nonspecific urethritis was 14% and that for pregnant women 71%. A very high titer (greater than or equal to 256) was recorded in 25 infertile women and in 7% of pregnant women. Among women with positive chlamydial isolation, 46% had titers greater than or equal to 256. Bilateral tubal obstruction was demonstrated in 23 infertile women (18%). The geometric mean titer of these patients was higher than that of patients with normal hysterosalpingograms. In nine cases with bilateral tubal obstruction and very high chlamydial antibody titers, there was no history of pelvis infections. The operative findings were typical of previous salpingo-oophoritis.