The MIC of gemifloxacin was compared with that of sparfloxacin, levofloxacin, moxifloxacin, gatifloxacin, ciprofloxacin, doxycycline, erythromycin, azithromycin and clarithromycin using 97 clinical isolates of Mycoplasma pneumoniae. MBCs of fluoroquinolones were determined for a subgroup of 12 isolates. Macrolides were the most potent agents with MIC(90)s for all drugs <or=0.001 mg/l. The doxycycline MIC(90) was 0.5 mg/l. Gemifloxacin MICs ranged from <or=0.001 to 0.25 mg/l. The gemifloxacin MIC(90) (0.125 mg/l) was equivalent to moxifloxacin and gatifloxacin, was 2-fold lower than sparfloxacin, 8-fold lower than levofloxacin and 32-fold lower than ciprofloxacin. MBCs for gemifloxacin were predominantly within 2-4 times the corresponding MIC values, indicating a bactericidal effect.