Pyrazinamidase activity in clinical isolates of Mycobacterium tuberculosis has been previously found to correlate with susceptibility to the antituberculosis drug pyrazinamide. The Wayne method for determining pyrazinamidase activity, a technique also utilized as an aid in identification of mycobacteria, and thin-layer chromatography method were found to be useful screening methods for susceptibility testing, since resistant strains are pyrazinamidase negative. These simple methods overcome the difficulty in growing M. tuberculosis at pH 5.5, as is required in the conventional method of susceptibility testing.