The specific catalase activities, after rupture of the cells of BCG, Mycobacterium smegmatis, and M. phlei were 2.2 to 3.5, 0.2 to 3.0, and 1.7 L X g-1 X min.-1, respectively; the peroxidase activities (PZ values) were 0.14 to 0.36, 0.034 to 0.20, and 0.17, respectively. Fe, Zn, and Co were necessary for maximal catalase and peroxidase levels. Most of the catalase in M. smegmatis and BCG is intracellular. M. smegmatis is freely permeable to H2O2, while BCG seems relatively impermeable. The peroxidase oxidized polyhydroxyphenols, but not monophenols, guaiacol, aromatic amines, or uric acid. The catalase and peroxidase were partially inhibited by 10-3 M azide, and by heating to 60[degree]C. for ten minutes. Both had optimal activity near pH 7.5, and both were precipitated by 3 volumes of ethanol. Both activities may belong to the one enzyme. The degree of inactivation of catalase by isoniazid depended upon its origin, the concentration of isoniazid, the duration and temperature of exposure, the oxygen tension, and the metal ions present. It is suggested that: inactivation results from the metal-catalyzed autoxidation of isoniazid with the formation of free radicals, probably hydroxyl (OH); inhibition of other enzymes may also be by this mechanism; and inhibition of M. tuberculosis is also through free-radical formation, a heme compound possibly being the catalyst.