Lurie et al. ' have found that administration of thyroid hormones can increase the level of native resistance to tuberculosis in rabbits, and that thyroidectomy and antithyroid drugs have the opposite effect. On the other hand, the harmful effects of iodides on the clinical manifestations of human leprosy are well known. 0 'Byrne2 and Roj as' have reported favourable responses in the treatment of this disease with thiouracil and methimazole . These reports would indicate a substantial difference in the effects of thyroid-related compounds on two infections produced by bacteria which are so closely related taxonomic­ ally. To further investigate these relations we treated mice with thyroid-related compounds and then infected them with another closely related mycobacteria, the Stefansky bacillus . Seventy albino-swiss mice, 7 weeks old, were divided into 4 groups and were treated as shown in the Table 1 .