SummaryEvidence is growing that high-molecular-weight glycoproteins (mucins) of human milk bind to certain pathogenic microorganisms and thus could interfere with their colonization of the infant gut. This investigation sought to determine whether one of the two principal milk mucins, MUC-1 (also known as PAS-0 and episialin), persists into the feces. Fecal samples from seven breast-fed and seven formula-fed infants, aged from 20 days to 6 months, were analyzed. Glycoproteins in the specimens, solubilized with sodium dodecylsulfate, mercaptoethanol, and heat, were resolved by gel electrophoresis. The gels were blotted on nitrocellulose sheets that were immunostained with monoclonal antibodies specific for epitopes in the tandem repeat region of MUC-1. Mucin fragments of ∼200,000 MW were detected in specimens from three of the seven breast-fed subjects; only one of them showed evidence of the intact mucin. Four breast-fed and all seven formula-fed babies were negative for the mucin in their stools. While these results provide evidence of a variable degree of breakdown, we speculate that MUC-1 resists degradation in the gut and thus may help bind host-detrimental microorganisms and inhibit their colonizing and infecting actions.