Oxythiamine, the 4''-OH analog of thiamine, is an effective thiamine antagonist in the nutrition of the chick. By increasing the level of oxythiamine fed, the chick wt. was decreased until typical thiamine-deficiency symptoms occurred and death ensued. Addition of large amts. of thiamine prevented the toxicity, and the parenteral or oral admn. of thiamine to oxythiamine-toxic chicks tended to overcome the inhibition. Evidence indicates that the thiamine requirement of growing White Leghorn cockerels is greater in the absence of a sufficiency of the unidentified chick growth factors present in crude casein, liver and fish meal.