Sialic acid binding proteins, sialic acid-binding peptides and sialic acid containing oligo-saccharides are active ingredients of a novel bacterial enterotoxin neutralizer and interfere with the binding of enterotoxins, including cholera toxin, to receptors so as to exert an effect of neutralizing the toxicity, thus being utilized as an effective neutralizer and are safe and available cheaply and abundantly from by-products produced during the processing of cow's milk which renders the enterotoxin neutralizer economically advantageous for commercial use.