Fucosylated oligosaccharides of human milk protect suckling mice from heat-stabile enterotoxin of Escherichia coli. Human milk protects suckling mice from the diarrheagenic effects of heat-stabile enterotoxin of Escherichia coli (ST). To identify the human milk fraction responsible for this protection, pooled skimmed, deproteinated milk was passed through charcoal, whereupon lactose was separated from the oligosaccharides. The oligosaccharides contained ST-protective activity; the lactose did not. The neutral, but not the acidic, fraction exhibited protective activity against ST (22% vs. 57% mortality, respectively; P less than .001). The fucosylated, but not the nonfucosylated, subfractions of the neutral fraction contained the factor protective against ST (35% vs. 50% mortality, respectively; P less than .05). An oligosaccharide isolation scheme based on different principles produced confirmatory results. The commercially available neutral fucosylated oligosaccharides of human milk did not significantly protect the mice from the effects of ST. Thus, the protective factor against ST seems to be a minor neutral fucosyloligosaccharide of human milk. 