The electrolyte, lactose, total protein and lipid composition of neonatal milk were examined. Electrolyte values were similar to those found in maternal milk; the Na concentration of milk from 1 infant with adrenal insufficiency and 2 infants with cystic fibrosis were normal. Inflammation of the breast was associated with high Na and low K values. The concentrations of lactose and total protein were similar to maternal milk. Neonatal milk contained significant amounts of fat with a full range of fatty acids. Total fat concentration was less, but the relative amount of shorter chain fatty acids was greater than in maternal milk. Milk from malnourished newborns contained less fat than the secretion of healthy infants. Total milk fat level appeared to increase with postnatal age. The neonatal breast produces a complex secretion, concluding that neonatal milk is indeed a true milk. The study of neonatal milk sheds light on the physiology of lactation and on metabolic processes in the newborn.