Breast milk is the best nutrient source for infants. It contains all elements needed for a normal growth and development of infants. Human milk contains a large amount of sialic acid compared with bovine milk. Sialic acid contained in oligosaccharides, glycolipids and glycoproteins in milk is considered to play important roles in physiological functions in infancy. Thus, we have investigated the sialic acid composition and the functions of sialylated compounds in human milk. Sialic acids comprise a family of neuraminic acid derivatives present in secretions, fluids and tissues of mammals. In milk, sialic acid is present in different sialoglycoconjugate compounds such as oligosaccharides, glycolipids and glycoproteins, not in a free form. Human milk contains 0.3-1.5 mg/ml of sialic acid. Sialic acid bound to oligosaccharides accounts for about 75% of the total sialic acid contained in human milk. Most of the sialic acid contained in human milk is found in the form of sialyllactose, an oligosaccharide formed from lactose and sialic acid. In milk, gangliosides, sialic acid-containing glycolipid, occur mainly as monosialoganglioside 3 (GM3) and disialoganglioside 3 (GD3). The concentration of GM3 in human milk increases, while that of GD3 concentration decreases during lactation. Because the brain and central nervous system contain considerable level of sialic acid in infancy, it is considered to play important roles on the expression and development of their functions. Moreover, we found that some sialylated compounds had inhibited the adhesion of toxins, bacteria and viruses to the receptors on the surface of epithelial cells. Additionally, we found that some sialylated compounds had growth-promoting effects on bifidobacteria and lactobacilli, predominantly present in the intestinal flora of infants fed with human milk. The results suggested that sialylated compounds in human milk possibly behaved as a physiological component in the intestinal tract of infants to protect them against enteric infections.