The majority of infants obtain total nutrition from milk during the early weeks or months of life. It seems logical, therefore, that knowledge of the usual composition as well as the variation in makeup of this biologic fluid should be of prime concern to those supervising the nutrition of infants. For this reason the memorandum which follows has been prepared by the Committee on Nutrition of the American Academy of Pediatrics. Milk is not constant in composition from one human or animal to another, at all periods of lactation, or even hourly through the day. Unqualified statements about human or cow milk and comparison of the milks of different species may, therefore, be somewhat misleading when specific samples of milk are considered. It is fortunate that a significant and comprehensive literature is available concerning the variations to be expected. SOURCES OF INFORMATION CONCERNING HUMAN AND COW MILK A list of the major sources of such information is given here. These references are concerned with human milk or with cow milk as compared to human milk. Detailed data on cow milk are available in the extensive literature of the dairy industry.1,2 A series of 21 papers on the secretion and composition of both human and cow milk, including an extensive bibliography has been published in the British Medical Bulletin.3 Macy, I. G.: Composition of human colostrum and milk.4 Kon, S. K. and Mawson, E. H.: Human Milk. Wartime Studies of Certain Vitamins and other Constituents.5 This report presents 12 studies and a summarizing conclusion on the fat and vitamin content of milk.