To evaluate the effect of the growth promoting activity in human milk on intestinal cells, a bioassay method was established using a fetal intestinal cell line (FHS 74 Int, ATCC CCL 241), since the developing intestine is considered to be a target organ for the growth factors present in human milk. Human milk had a growth promoting activity on the cultured human fetal intestinal cells. The activity level was very high in colostrum and decreased gradually during lactation, while formula products had no activity. The epidermal growth factor (EGF) concentration in human milk was significantly correlated with the growth promoting activity measured by bioassay. Thus, EGF may be the main growth factor for the proliferation of intestinal cells. These results suggest that human milk may stimulate the proliferation of intestinal cells in newborn infants, especially in very-low-birth-weight infants, and accelerate the maturation of the intestinal portion of the digestive system.