Forty isoniazid-resistant Mycobacterium tuberculosis isolates were characterized on the basis of phenotypic properties (i.e., catalase activity, MIC of isoniazid, and growth pattern in the presence of 7 different concentrations of isoniazid) and alterations in the katG gene (codons 315 and 463). Three different growth patterns could be distinguished: concentration-dependent inhibition of growth was observed in 29 strains, similar growth at all concentrations was seen in 7 strains, and enhanced growth at low concentrations of isoniazid was evident in 4 strains. The MIC of isoniazid was < or = microg/ml for 29 of 40 strains. Mutation at codon 315 of the katG was detected in 28 of 40 strains. However, only one of the seven strains for which the MIC of isoniazid was > or = 16 microg/ml had mutation at this codon. Five of these seven strains for which the MIC was > or = 16 microg/ml had no catalase activity. The results indicate that the MIC of isoniazid for a majority of strains is below the level achievable in serum. Therefore, isoniazid may be beneficial for the treatment of some cases of multidrug-resistant tuberculosis. Determination of catalase activity aids in the detection of isolates for which MICs are high and could, in conjunction with molecular methods, provide rapid detection of most isoniazid-resistant strains.