Forty-six men were successfully treated with a single oral dose of ampicillin (2 g) plus probenecid (1 g) for urethral infections with Neisseria gonorrhoeae. Chlamydia trachomatis was isolated from cultures of cells obtained from 11 of these men both before and after treatment; C. trachomatis was isolated from one man before but not after treatment and from three men after but not before treatment. No isolates were obtained from the remaining 31 men either before or after treatment. Of the 15 patients whose cultures yielded C. trachomatis, 12 developed postgonococcal urethritis; of the 31 patients from whose cultures no isolate was obtained, five developed postgonococcal urethritis. Of 44 women successfully treated with ampicillin plus probenecid for cervical infections with N. gonorrhoeae, 18 had C. trachomatis isolated from the cervix both before and after treatment. C. trachomatis was isolated from five women before but not after treatment and from two women after but not before treatment. No isolates were obtained from the remaining 19 women either before or after treatment. Thus ampicillin plus probencide in the dosage used rarely eliminated C. trachomatis from the genital tract of either men or women. Whereas men with a persisting chlamydial infection will probably develop postgonococcal urethritis and thus receive appropriate treatment, such an infection in women is not likely to be suspected unless attempts are made to isolate C. trachomatis.