The reliability of mycobacteria growth indicator tubes (MGIT) for susceptibility testing of Mycobacterium tuberculosis to isoniazid and rifampin was evaluated by comparing MGIT results to those obtained by the radiometric BACTEC TB system. To resolve discrepancies, MGIT and BACTEC TB tests were repeated, and the method of proportion was performed. For the 89 isolates evaluated, there was complete agreement between MGIT and BACTEC results for rifampin: 70 isolates were susceptible, and 19 were resistant. After initial testing of isoniazid, both methods agreed for 85 isolates (95.5%): 72 were susceptible, and 13 were resistant by both methods; 2 were resistant by BACTEC but susceptible by MGIT, and 2 were resistant by MGIT but susceptible by BACTEC. Based on results of discrepant analysis for the latter four isolates, all four were resistant to isoniazid. Thus, MGIT correctly detected susceptibility and resistance to isoniazid for 97.3% and 100% of the isolates, respectively. The mean time to MGIT results was 6.38 +/ 0.11 days (range, 6 to 14 days), compared to a mean of 9.09 +/- 1.90 days (range, 4 to 12 days) for BACTEC (p < .001). MGIT provides a rapid and reliable, nonradiometric alternative to BACTEC for susceptibility testing of M. tuberculosis to isoniazid and rifampin.