Three different types of isoniazid-resistant mutants were identified in parent drug-susceptible populations of 3 strains of virulent tubercle bacilli. Two of these mutants were markedly deficient in catalase activity. Because pyruvic acid and hemin under ideal conditions, especially with Mn, may be antagonistic to isoniazid, studies were made to determine the possible antagonistic activities of inorganic Fe, hemin, and pyruvate. None of these substances showed sufficient antagonistic activities to account for the differential growth requirements of the isoniazid-susceptible parent strain and the 2 isoniazid-resistant mutants. The 3d type of mutant was not so highly deficient in catalase activity.