The in vitro activities of BMS-284756 (a novel des-fluoroquinolone), levofloxacin, moxifloxacin, and clarithromycin were tested against 5 strains of Chlamydia trachomatis and 20 isolates of Chlamydia pneumoniae. The MIC at which 90% of the isolates were inhibited and the minimal bactericidal concentration at which 90% of the isolates were killed by BMS-284756 for all isolates of C. pneumoniae and C. trachomatis was 0.015 microg/ml (range, 0.015 to 0.03 microg/ml). BMS-284756 was the most active quinolone tested.