Indirect immunofluorescence (the micro IF test) was used to study trachomainclusion conjunctivitis and lymphogranuloma venereum (LGV) serum antibody patterns in a venereal disease clinic population. Antibodies undifferentiated as to immunoglobulin class, but probably IgG, were detected in 80% of the study participants and titers were particularly high in LGV patients. Two kinds of antibody were observed. One was broadly reactive to all test antigens, and one was specifically directed toward strains of a particular antigenic type. Although most sera were broadly reactive, 40% also showed some type specificity. Specific antibody patterns in persons from whom chlamydiae were isolated usually corresponded to the immunotypes of isolates, and LGV reaction patterns were particularly associated with LGV patients. IgM antibodies were observed in 20% of the study participants. These were usually type specific and perhaps indicated current infection. The sensitive micro IF test is considered useful in the epidemiologic study of chlamydial infections of genital origin and can provide information about the nature of infecting strains.