Twenty-one oligosaccharides of human milk were quantified by high-performance anion-exchange chromatography. Milk samples were collected from 18 mothers during the first 3 mo of lactation. The data show that the highest amount of all oligosaccharides is present at day 4 postpartum (20 g l(-1)) and then decreases by about 20% at day 30 of lactation. The protective role played by these substances against different infectious agents, in different organs and systems of the breastfed baby, is emphasized.