Human milk contains a wide variety of nitrogenous compounds in addition to protein. Recognition of the special roles these compounds can perform raises questions about their availability from human milk, and, ultimately, their significance in the development of the human newborn. While it is likely that the major categories of compounds contributing to the nonprotein-nitrogen fraction of human milk have been identified, the true variety of nitrogenous compounds within the peptide fraction of human milk is only beginning to be recognized and appreciated. If predictions can be made from those peptides already identified, epidermal growth factor, delta-sleep inducing peptide, somatomedin-C/insulin-like growth factor I and the peptide hormones, further elucidation of the specific peptides that contribute to this fraction of human milk promises to be especially exciting. With each new published report, the recognized chemical gap between human milk and proprietary formulas increases. There is increasing evidence that human milk produced by a well-nourished woman is a chemical mixture uniquely suited for the developmental stage of her infant. Whether these differences confer developmental advantages to the infant fed human milk, advantages not enjoyed by infants fed formula, is less easily determined. Attempts to answer this question must take into account the relative physiological maturity of the infant at birth. There is a distinct possibility that infants born early in the last intrauterine trimester will derive more benefit from receiving mother's milk than those infants nourished in utero to term.