We evaluated the in vitro antimicrobial activity of T-3761 and ofloxacin against representative pathogenic mycobacteria by the agar dilution method, using 7H11 agar medium. T-3761 showed appreciable antimicrobial activity against Mycobacterium tuberculosis (MIC90: 3.13 micrograms/ml), M. kansasii (MIC90: 6.25 micrograms/ml) and M. fortuitum (MIC90: 3.13 micrograms/ml), whereas the agent was not active against M. marinum (MIC90: 25 micrograms/ml), M. scrofulaceum (MIC90: 50 micrograms/ml), M. avium (MIC90: > 100 micrograms/ml), M. intracellulare (MIC90: > 100 micrograms/ml), M. chelonae subsp. abscessus (MIC90: > 100 micrograms/ml) and M. chelonae subsp. chelonae (MIC90: 50 micrograms/ml). The in vitro antimicrobial activity of T-3761 against M. fortuitum was a little more potent than that of ofloxacin, whereas the activity of T-3761 against the other mycobacteria was slightly inferior to that of ofloxacin. The antimycobacterial activity of T-3761 against M. tuberculosis H37Rv (MIC in vitro: T-3761 = 3.13 micrograms/ml, OFLX = 0.78 micrograms/ml) phagocytosed in murine peritoneal macrophages was less active than that of ofloxacin, when the macrophages were cultured in RPMI-1640 medium containing 1 microgram/ml or 10 micrograms/ml of these agents for up to 5 days. The activity of 10 micrograms/ml of T-3761 was equivalent to that of 1 microgram/ml of ofloxacin.