Anti-EGF serum administered to newborn mice led to an increasing rate of growth retardation and body weight gain during the following period up to 5 weeks after birth, but sometimes to an effect found reversible after 3-4 weeks. In addition, striking delay of differentiation of skin epidermis and appendages and of intestinal Paneth's cells was ascertained about 4 weeks after birth. The findings point to neutralization of EGF or abolition of its effects by anti-EGF serum and, thus, substantiate the suggestion that EGF plays a physiological role during the early postnatal growth and differentiation.