The in-vitro activity of moxifloxacin, a new 8-methoxyquinolone, was compared with minocycline and azithromycin against 40 strains of Chlamydia trachomatis, Chlamydia pneumoniae and Chlamydia psittaci. Both the MIC and the MBC of moxifloxacin ranged from 0.03 to 0.125 mg/L. MICs of minocycline ranged from 0.015 to 0.06 mg/L and MBCs between 0.03 and 0.25 mg/L. MICs of azithromycin ranged from 0.03 to 0.125 mg/L and the MBCs between 0.06 and 0.5 mg/L. MBC values of moxifloxacin were the same as MICs in 32 (80%) of 40 strains tested, whereas those of minocycline and azithromycin were two to four times higher than their MICs. These data confirm those previously obtained indicating that quinolones kill chlamydial strains at concentrations equivalent to their MICs.