The reliability of mycobacteria growth indicator tubes (MGIT) for determining the susceptibility of Mycobacterium tuberculosis to isoniazid (0.1 microgram/ml) and rifampin (2.0 micrograms/ml) was evaluated by comparing MGIT results to those obtained by the radiometric BACTEC TB method. We tested 29 isolates, many selected for resistance. The turnaround times were 3-8 days (median 6) for MGIT and 4-10 days (median 6) for BACTEC. Isoniazid results by both methods agreed for all isolates tested: 20 resistant and nine susceptible. Rifampin results agreed for 28 isolates: 10 resistant and 18 susceptible. One isolate yielded discrepant results: resistant to rifampin by BACTEC, but susceptible by MGIT. This isolate was also rifampin-resistant by the traditional Method of Proportion and when tested by the MGIT method using 1.0 micrograms/ml of rifampin. The MGIT system is a nonradiometric alternative to the BACTEC for rapid susceptibility testing of M. tuberculosis to isoniazid and rifampin; however, additional testing is needed to determine the optimal concentration of rifampin.