The sialoglycoconjugate content of human milk has been extensively studied. However, little attention has been paid to the changes occurring in these compounds in bovine milk during lactation. Since sialoglycoconjugates are very abundant in milk from the early stages of lactation, they have been suggested to be important for the nutrition of the newborn during the first months of life. The distribution of sialoglycoconjugates (expressed as glycoconjugate-bound sialic acid) from four different stages of lactation (colostrum, transitional, mature, and late-lactation milks) was investigated in four Spanish-Brown cows. All the fractions studied (total sialic acids, glycoproteins, oligosaccharides, casein, and gangliosides) showed a similar trend. We found the highest values in the colostrum, these decreasing in transitional and mature milks and increasing again in late-lactation milk. We also found a selective change in the relative contents of glycoprotein- and oligosaccharide-bound sialic acids. In mature milk, the latter increased up to 80% (59% in colostrum) and the former decreased to 3.9% (35.3% in colostrum). It would appear that the decrease in oligosaccharide-bound sialic acid is compensated by the increase in glycoprotein-bound sialic acid. From these results, it is deduced that newborn infants or calves fed with infant formulas or milk replacers, respectively, should be supplemented with sialoglycoconjugates to approximate the composition of human and cow milk as far as is practicable.