The in vitro susceptibilities of 49 strains of Chlamydia trachomatis and 3 strains of Chlamydia pneumoniae to azithromycin and tetracycline or doxycycline were determined. The MIC of azithromycin ranged from less-than-or-equal-to 0.06 to 1.0 mug/ml, the MIC of tetracycline ranged from 0.03 to 0.12 mug/ml, and the MIC of doxycycline ranged from 0.015 to 0.06 mug/ml against C. trachomatis. The MIC ranges for C. pneumoniae were 0.12 to 0.25 mug/ml for azithromycin and 0.06 to 0.12 mug/ml for tetracycline. All minimal chlamydicidal concentrations were either equal to the MIC or one or two dilutions higher. No strains resistant to these antibiotics were detected. In vitro activity shows that azithromycin is highly active against C. trachomatis and C. pneumoniae.