MICs of ciprofloxacin were lower than MICs of ofloxacin for both M. tuberculosis and M. avium complex strains. The MICs of both drugs for 41 M. tuberculosis strains were in a narrow range, and significantly lower than the achievable serum concentrations. The MICs of ciprofloxacin for 46 M. avium strains were in a wide range, and in only 28% were the broth-determined MICs of ciprofloxacin within the same range as for M. tuberculosis and lower than the achievable serum level. When compared with the concentrations achievable in macrophages, the broth-determined MICs of ciprofloxacin were lower than this level for 61% of M. avium strains. Both drugs were bactericidal against M. tuberculosis and M. avium, with a low MIC/MBC ratio.