Activities of newly synthesized fluoroquinolnes WQ-3345 and WQ-3402 against M. leprae were measured by using the Buddemeyer method. The % inhibition of the examined drugs for M. leprae was in the order of RFP > WQ-3402 > SPFX > GFLX > WQ-3345 > LVFX. The anti-M. leprae activity of WQ-3402 was found to be strongest in these five fluoroquinolones when examined by this method, and the activity of WQ-3345 was weaker than that of GFLX. The anti-M. leprae activities of WQ-3345 and WQ-3402 were measured by a mouse footpad method using nude mice. The inhibitory effects on the growth of M. leprae inoculated into the footpads were found to be incomplete after orally administered with WQ-3345 or WQ-3402 respectively at dosages of 10 and 20 mg/kg, and the incomplete inhibition was again was again found even at a dosage of 30, 40, or 50 mg/kg im the latter.