The in vitro activities of 20 fluoroquinolones against Mycobacterium leprae were evaluated by using the BACTEC 460 system. M. leprae was incubated in BACTEC 12B medium at 33 degrees C under reduced oxygen for 2 to 3 weeks in the presence of fluoroquinolones at 0.31 to 5 micrograms/ml. Activity was determined by a reduction in 14CO2 evolution compared with that of drug-free controls. Of the commercially available agents, ofloxacin was most active, while enoxacin and norfloxacin were inactive. However, a number of newer fluoroquinolones (AT-4140, OPC-17100, OPC-17066, PD-117596, PD-124816, PD-127391, and WIN-57273), all containing a cyclopropyl group at R-1 and, with the exception of WIN-57273, either a halogen or methyl group at R-8, were more active than ofloxacin in vitro. Further in vivo evaluations of these agents should help determine their potential for use against leprosy.