During a two-year period, sera from 1179 patients with urogenital tract symptoms and from 256 controls were tested for IgG and IgA antibodies to Chlamydia trachomatis, using a commercially prepared indirect immunoperoxidase assay. Of the patient sera, 1051 (89%) were from female patients and 128 (11%) were from males. Specimens were received from patients attending private practitioners (424), government polyclinics (348), the Queen Elizabeth Hospital (206), the Barbados Family Planning Association (117) and various other sources (84). Nine hundred and fifty-nine of specimens (81%) from all patients had IgG titres of 64 or greater, and 441 (37%) had IgA titres of 16 or greater. A lower proportion of male patients (43%) than female patients (61%) had IgG titres of 128 or greater. Among 76 control females attending the antenatal clinic, IgG was detected in 76% and IgA in 16%, while among 75 adult blood donors, IgG was detected in 70% and IgA in 39%. A group comprising 97 asymptomatic children aged 9 months to 13 years, and 8 adolescents aged 14-18 years was also studied. IgG antibodies (titre > or = 64) were detected in 13.4% of children and in 50% of the adolescents. IgA antibodies were detected in only 1% of children and adolescents. Antibodies were not detected in children under the age of 5 years. The detection of antibodies by indirect immunoperoxidase assay in such a high proportion of the population suggests that this method is of little value for diagnosis of infection with Chlamydia trachomatis.