The authors compared conventional and Bactec determinations of pyrazinamide susceptibility with pyrazinamidase activity determined using either standard Dubos Oleic agar or Middlebrooks 7H9 agar. Of the 26 Mycobacterium tuberculosis tested, Bactec found 16 strains susceptible and 10 resistant, conventional methods found six susceptible and 20 resistant. Dubos identified 13 and Middlebrooks 10 strains as producing pyrazinamidase (susceptible). Of the 11 strains susceptible in Bactec and resistant by conventional methods, eight produced pyrazinamidase. All mycobacteria other than tuberculosis (6) and Mycobacterium bovis (2) were resistant by all methods. Bactec results gave a better correlation with pyrazinamidase results than conventional tests.