The M-1 acidic glycoprotein fractions isolated from individual samples of bovine colostrum and milk whey, although homogeneous by ultracentrifugal and boundary electrophoresis criteria, contained several N-terminal amino acids and, in some instances, several bands on gel electropherograms and, thus, probably consisted of a series of closely related molecular species. Their average molecular weights were near 10,000, with high β-values and negative specific rotations, indicating highly disorganized structures. Amino acid analysis showed high percentages of glutamic acid, proline, threonine, and isoleucine, and unusually low amounts of basic amino acids. Monosaccharides in the colostrum M-1 glycoprotein fractions were galactose, glucosamine, galactosamine, and sialic acid, the latter being easily removable by viral neuraminidase. The only hexose detected in the milk M-1 glycoprotein fraction was galactose.