Sixty-three heterosexual men were successfully treated with a single injection of spectinomycin hydrochloride 2 g for urethral infections with Neisseria gonorrhoeae. Chlamydia trachomatis was recovered from the urethra of 11 of these men both before and after treatment. In six men, the organism was isolated after but not before treatment. No isolates were obtained from the remaining men either before or after treatment. All 17 of the men who yielded C. trachomatis developed post-gonococcal urethritis. Eight of 46 men from whom no isolate was obtained in their cultures developed post-gonococcal urethritis. Seventeen of 50 women successfully treated with spectinomycin for cervical infections with N. gonorrhoeae yielded isolates of C. trachomatis both before and after treatment. The organism was isolated from five women before but not after treatment, and from four women after but not before treatment. In 24 women culture for C. trachomatis was negative both before and after treatment. Spectinomycin hydrochloride in the dosage used rarely eliminated C. trachomatis from the genital tract of either men or women; in this respect it resembled two other drugs commonly used for the treatment of gonorrhoea-pencillin and ampicillin.