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SUMMARY: In confirmation of Fisher’s (1952, 1954a, b) results, haemin was found to act as an apparent growth factor for isoniazid-resistant strains of tubercle bacilli and also powerfully to antagonize the action of isoniazid on drug-sensitive organisms. In extension of Fisher’s work, these effects of haemin were investigated with drug-sensitive and resistant strains of Mycobacterium tuberculosis H37 Rv and B.C.G. The effect of Tween 80 and albumin was also examined. In Fisher’s medium when the nitrogen source was changed isoniazid-resistant strains no longer appeared to be haemin-dependent. Middlebrook’s observations that isoniazid-resistant tubercle bacilli are deficient in catalase (Middlebrook, 1954) and that catalase enables resistant organisms to grow in a deficient medium (Middlebrook, personal communication) were confirmed. Since the dependence of isoniazid-resistant organisms on haemin or catalase seems itself to be dependent on the presence of asparagine in the medium, it is suggested that haemin has a catalase or peroxidase-like role, and that in the presence of asparagine isoniazid-sensitive organisms can remove peroxide, whereas isoniazid-resistant organisms cannot.