Isoniazid interferes with some phase of the metabolism of the tubercle bacilli involving pyridoxal. Due to the similarity in the chem. structure of isoniazid and pyridoxal, a true antagonism of the competitive type is suggested between isoniazid and this essential metabolite. Antagonism of isoniazid was also obtained with alpha-keto-glutarate, Na pyruvate, and pyridoxamine.