Sera obtained from 34 men with Chlamydia trachomatis positive non-gonococcal urethritis, 34 men with C trachomatis negative non-gonococcal urethritis, 42 women with acute salpingitis, 38 healthy women, and 34 healthy men were studied for the presence of specific serum C trachomatis IgA and IgG antibodies. Serological results were correlated with C trachomatis isolation in cell culture. An enzyme linked immunosorbent assay (ELISA) for C trachomatis specific serum IgA was employed using highly purified elementary bodies of C trachomatis serotype L2 grown in LLC-MK2 cells. Results obtained for C trachomatis IgA antibody by the ELISA test were compared with results obtained for the same sera by a single antigen immunofluorescence technique. A good correlation (r = 0.91) was found between two methods. Serum IgG antibody was also determined in the same sera by the immunofluorescence technique. Patients with C trachomatis positive non-gonococcal urethritis had a significantly (p less than 0.0005) higher prevalence (94.1%) of serum IgA antibody by ELISA compared with patients with C trachomatis negative non-gonococcal urethritis (20.5%) or healthy men (5.9%). Similarly, women with acute salpingitis had a significantly (p less than 0.005) higher prevalence of serum IgA antibody (45.2%) compared with healthy controls (5.2%). Comparable results were obtained for C trachomatis serum IgA antibody using the immunofluorescence technique. The prevalence of C trachomatis IgG antibody was significantly higher in patients with C trachomatis positive non-gonococcal urethritis (97.0%) compared with those with C trachomatis negative non-gonococcal urethritis (33.3%) and healthy controls (23.5%). The importance of using specific C trachomatis serum IgA in the identification of chlamydial infection is discussed.