Growth factor activity is present in human milk throughout the lactation period but is in bovine milk only during the colostral phase. Biochemical analysis shows that the growth factors in human and bovine milk are not the same. Human milk contains three species of growth factors. One of these, designated human milk growth factor (HMGF) III, constitutes over 75% of the total growth factor activity of human milk. HMGF III has a mol wt of about 6,000, a pI between 4.4 and 4.7, and is resistant to treatment with dithiothreitol. Comparative biochemical studies strongly suggest that HMGF III is a human epidermal growth factor (EGF). On the other hand, bovine colostrum has no EGF-like growth factor. Instead, the major growth factor component of bovine colostrum has a mol wt between 30,000 and 35,000 and is inactivated by dithiothreitol treatment. The bovine colostrum growth factor (BCGF) is similar biochemically to one of the minor growth factors in human milk, HMGF II, which accounts for about 20% of the growth factor activity of human milk. HMGF III, the EGF-like polypeptide, can be purified to homogeneity by a combination of size exclusion and anion exchange chromatography. Purified HMGF III stimulates DNA synthesis in 3T3 cells at a concentration of about 25 ng/ml.