The problem of the reenforcement of milk with vitamin D has assumed increasing importance in recent years, since this type of milk in its various forms is being marketed widely throughout the country. In spite of numerous studies, pediatricians and others having to do with the feeding of infants are in doubt as to the extent to which this type of milk can protect a growing infant from rickets. The majority of studies have dealt with the problem of curing rickets already established. Up to the present, few studies have been reported on the prophylactic potency of this type of antirachitic agent.A comprehensive review of the reports of clinical experiences with vitamin D milk published prior to October 1934 was made by Bunker and Harris.1 They expressed the opinion that irradiated milk and milk from cows fed irradiated yeast had each proved to be valuable both in the cure and in the prevention of rickets when employed in the amounts used and in the manner usually applied with ordinary milk in the feeding of infants. In the experiments reviewed no constant difference had been demonstrated in the relative efficacies of these two types of milk in terms of potency according to rat unit equivalents. They indicated a number of problems remaining unsolved : the special requirements of very young infants in respect to antirachitic factors ; the minimum antirachitic potency of milk required for complete protection of all infants, and the quantity of vitamin D needed to insure the greatest rapidity of the healing process.