The experiments reported in this paper are directed at a possible antimetabolite relationship between isoniazid and pyridoxine. The effects of vitamin B6 deficiency alone, vitamin B6 deficiency plus isoniazid and vitamin B6 deficiency plus desoxypyridoxine administration were observed. The symptoms of pyridoxine deficiency were aggravated by both isoniazid and desoxypyridoxine but the two compounds produced different effects on certain of the manifestations of vitamin B6 deficiency. The paper is of considerable theoretical interest in demonstrating that antimetabolites may differ in the intensity that they affect the functions of the compound with which they are in conflict. The results further emphasize the possible clinical significance of vitamin B6 deficiency occurring in patients undergoing isoniazid therapy for tuberculosis. The toxic effects of isoniazid which simulate vitamin B6 deficiency may be prevented by assuring an adequate intake of vitamin B6 during therapy. Amounts of vitamin B6 which meet the daily requirements do not appear to affect the antituberculosis activity of isoniazid.