Human milk contains a variety of complex lactose derived oligosaccharides in relative high amounts which are only minimally digested during their passage through the orogastrointestinal tract [Gnoth, M. J., Kunz, C., Kinne- Saffran, E., & Rudloff, S., (2000a). Human milk oligosaccharides are minimally digested in vitro. Journal of Nutrition, 130, 3014-3020] Since these components were detected in the urine of breast-fed infants [Obermeier, S., Rudloff, S., Pohlentz, G., Lentze, M. J., & Kunz, C. (1999). Section of 13(C)-labelled oligosaccharides into human milk and infant's urine after an oral [C-13]galactose load. Isotopes Environmental Health Studies, 352, 119-125], we investigated possible intestinal transport mechanisms using confluent Caco-2 layers grown on permeable membrane filter supports. When the apical side of these cell layers was exposed to HMO virtually the complete milk oligosaccharide pattern could be detected on the basolateral side. For neutral, but not for acidic oligosaccharides, the transport from the apical to the basolateral compartment was higher than in the opposite direction. Quantitatively, the transport from the apical to the basolateral compartment was 1-1.5% of the applied amount of oligosaccharides. (C) 2002 Elsevier Science Ltd. All rights reserved.