ExtractTwo milk-specific glycoprotein fractions and serum orosomucoid were isolated from mature human milk. One of the milk-specific glycoproteins was homogeneous and was termed glycoprotein A. Its sedimentation and diffusion constants were 2.6 S and 6.3 F, respectively, giving a molecular weight of 29,500. It contained some 70% carbohydrate consisting of sialic acid, galactose, fucose, glucosamine, and galactosamine. It was not immunologically related to serum orosomucoid. The other glycoprotein, called glycoprotein B, was heterogeneous with an average molecular weight of near 4,000, and carbohydrate comprised some 75% of its weight. Both glycoprotein fractions supported the growth ofLactobacillus bifidusvar.Penn.Removal of sialic acid enhanced the growth-promoting activity of glycoprotein B but not of A.SpeculationMature human milk whey contains a number of glycoproteins that stimulate the growth ofL. bifidusvar.Penn.These glycoproteins may act in concert with the well known oligosaccharides to maintain an acid environment in the infant's gastrointestinal tract.