Rotavirus is the major cause ef in£ectious diarrhea ill infants and young children al1 oyer the world. We haye found that a high-M, glycoprotein fraction from cow's milk is potent in inhibiting replicatioll of human rotayiruses in vitro. Since the actiyity seems to be unique and specific, this fraction may be useful as a novel agent for treatment or prevention of rotavirus diarrhea.