To achieve better sensitivity than direct testing and better turnaround time than current culture and identification methods, the Gen-Probe Mycobacterium Tuberculosis Direct method was used to detect Mycobacterium tuberculosis in BACTEC 12B medium cultures when they first gave a growth index (GI) of at least 10 (MTD/BACTEC method). Of 179 acid-fast, smear-positive specimens that were culture positive for M. tuberculosis, all were positive by the MTD/BACTEC method (sensitivity, 100%). Positive results were obtained only with tuberculosis patients. For diagnostic specimens from untreated patients, the mean time to achieve a GI of 10 was 6 days.