Fifty six clinical isolates of Mycobacterium tuberculosis were tested for drug susceptibility in Mycobacteria Growth Indicator Tube (MGIT) containing 0.1 microgram/ml of INH, 1.0 microgram/ml of RFP, 3.5 micrograms/ml of EB and 0.8 microgram/ml of SM. These results were compared with those obtained by testing the same M.tuberculosis isolates by the absolute concentration method using 1% Ogawa egg slant containing 0.1 microgram/ml of INH, 10 micrograms/ml of REP, 2.5 micrograms/ml of EB and 20 micrograms/ml of SM. Fifty six isolates consisted of 18 pansensitive strains, 27 multidrug resistant strains and 11 single drug resistant strains. The results of individual drugs showed excellent agreement between the MGIT and the Ogawa methods, and overall agreement rate of the two methods were 96.4%. The results were just the same for all drugs in 48 out of 56 strains studied. The drug resistance could be observed much earlier by the MGIT method (mean 5.9 days) than by the Ogawa method (more than 21 days). In conclusion, the MGIT system could be a promising new drug susceptibility test which might become available in Japan replacing the Ogawa method.