Moxifloxacin (BAY12-8039) is a new 8-methoxyquinolone shown to be active against Mycobacterium tuberculosis in vitro. We tested moxifloxacin for activity in mice against M. tuberculosis CSU93, a highly virulent, recently isolated clinical strain. The MIC of moxifloxacin for the CSU93 strain was 0.25 microg/ml. The serum moxifloxacin concentration after oral administration in mice peaked within 0.25 h, reaching 7.8 microg/ml with doses of 100 mg/kg of body weight; the maximum concentration and the analysis of the area under the concentration-time curve revealed dose dependency. When mice were infected with a sublethal inoculum of mycobacteria and then treated with moxifloxacin at 100 mg/kg per day for 8 weeks, the log10 CFU counts in the organs of treated mice were significantly lower than those for the control group (0.6 +/- 0.2 versus 5.6 +/- 0. 3 in the lungs and 1.5 +/- 0.7 versus 4.9 +/- 0.5 in the spleens, respectively; P < 0.001 in both organs). The effectiveness of moxifloxacin monotherapy was comparable to that seen in mice receiving isoniazid alone. Combination therapy with moxifloxacin plus isoniazid was superior to that with moxifloxacin or with isoniazid alone in reducing bacillary counts in the organs studied. Using a sensitive broth-passage subculture method, we demonstrated that 8 weeks of treatment with moxifloxacin (100 mg/kg per day) or with moxifloxacin plus isoniazid (100 mg/kg and 25 mg/kg, respectively, per day) sterilized the lungs in seven of eight and in eight of eight mice, respectively. Among surviving bacilli isolated from animals infected with a high-titer inoculum and treated for 7 weeks with low-dose moxifloxacin (20 mg/kg per day), breakthrough resistance to moxifloxacin was not observed. These results indicate that moxifloxacin is highly effective in reducing M. tuberculosis infection in mice and has activity comparable to that of isoniazid. Combination therapy with moxifloxacin and isoniazid was highly effective, suggesting that moxifloxacin may be useful in multiple-drug regimens for human tuberculosis.