Pyrazinamidase activity has been found to correlate with pyrazinamide sensitivity in strains of Mycobacterium tuberculosis. In vitro sensitivity to pyrazinamide in acidified Löwenstein-Jensen medium, and pyrazinamidase activity by the Wayne method, were determined in 378 clinical isolates of M. tuberculosis. A close correlation was observed between the results of both tests. This method of detecting pyrazinamidase activity was found to be a rapid, simple and reliable substitute for pyrazinamide sensitivity testing, and it overcomes the difficulty of growing M. tuberculosis at pH 5.5, as required in the standard method.