When applied to the chorio-allantoic membrane of the chick embryo, isoniazid was shown to produce an increase in the fragility of the embryo and in the amount of collagen which was extractable from the bones with cold 1 M sodium chloride. The administration of pyridoxal reversed these phenomena almost completely. The effect of isoniazid differed from that of beta-aminopropionitrile in that the latter was of greater magnitude, and was not affected by pyridoxal; whereas beta-aminopropionitrile caused skeletal deformities, isoniazid even at 12 times the concentration produced no deformities. The aldehyde group of pyridoxal was shown to be necessary for its interaction with isoniazid.