The faltering of growth which frequently occurs in babies at the age of about 3 months in developing countries is sometimes attributed to insufficient food or to increased exposure to infection. The adequacy of breast-milk as a sole source of food for the young infant is examined in relation to the calculated average requirements for protein and energy. It is concluded that breast milk will fail to cover the energy needs of many infants after about 3 months unless a regulatory mechanism permits infants with higher than average requirements to obtain larger amounts of milk.