A simple method is presented for producing large numbers of inclusions of Chlamydia trachomatis serotype L2 in cell cultures on slides for immunofluorescence antibody staining. Preliminary results with a total of 1,241 human sera from different groups were consistent with findings by earlier methods: 82% of chlamydia-positive men with nongonococcal urethritis had antibodies at titers of greater than or equal to 8, with a geometric mean titer (GMT) of 44.9; 68.5% of chlamydia-negative men with nongonococcal urethritis had titers of greater than or equal to 8, with a GMT of 38.6; 27% of male blood donors had titers of greater than or equal to 8, with a GMT of 19.6; 95.0% of chlamydia-positive women had titers of greater than or equal to 8, with a GMT of 80.3; 67.0% of chlamydia-negative partners of men with nongonococcal urethritis had titers of greater than or equal to 8, with a GMT of 47.0; 50.2% of control women had titers of greater than or equal to 8, with a GMT of 27.7% and 8.1% of children (aged 1 to 15 years) had titers of greater than or equal to 8, with a GMT of 17.8.