The effect of four intravaginal chemical contraceptives on the in vitro growth of Chlamydia trachomatis in cycloheximide-treated McCoy cells was studied. Encare produced significant inhibition, while Koromex had little antichlamydial activity. Both Intercept and Conceptrol had an intermediate effect, reducing chlamydial replication to 60-70% compared to untreated controls. Some variation in these effect against different strains of Chlamydia has been observed. In addition, the antichlamydial properties observed do not seem to be solely mediated by the pH of the preparations or the concentration of nonoxynol-9, the active ingredient in these contraceptives. Thus, intravaginal contraceptives may have a role in decreasing asymptomatic transmission of this organism and possibly as an adjunct to antibiotic therapy in symptomatic patients.