The oligosaccharide content of breast-milk and urine from ten nursing mothers and their babies, collected 30 days after delivery, was analysed by thin-layer and high-performance liquid chromatography. Each woman's milk and urinary oligosaccharide profiles were very similar, and the pattern of oligosaccharides excreted by her infant was also strongly correlated with that of her milk. The babies excreted 300-500 mg/day oligosaccharides and the mothers 500-800 mg/day. The effect of oligosaccharide fractions from a 500 ml pool of colostrum on bacterial adhesion to uroepithelial cells was tested on a strain of Escherichia coli isolated from an infant with urinary tract infection. The high-molecular-weight sialylated oligosaccharides had no effect but neutral oligosaccharides caused inhibition of bacterial adhesion which rose as the size of the oligosaccharides decreased. These findings suggest that breastfeeding may have a preventive effect on urinary tract infection in both mother and infant.