The in-vitro activity of a 3-azinomethyl-rifamycin (SPA-S-565) against Chlamydia trachomatis was compared to that of rifampicin and of erythromycin. The compound showed excellent activity for standard strains as well as isolates from patients with sexually transmitted diseases, being more active than the other drugs tested. SPA-S-565 also showed a low frequency of emergence of resistance. Passage of standard strains at sub-inhibitory concentrations caused an increase in MIC values of rifampicin while those for SPA-S-565 remained unchanged.