Epidermal growth factor is a polypeptide that stimulates proliferation and differentiation of a variety of cell types, including the developing intestinal epithelium; it is the agent in human milk that induces mitosis in human fibroblast culture. We systematically evaluated the EGF content of milk from 20 women delivering prematurely and from 11 women delivering at term. In preterm mothers, the concentration of EGF was 70 +/- 5 ng/ml (mean +/- SEM), with no significant change during seven weeks of lactation. EGF concentration in milk of term mothers was 68 +/- 19 ng/ml (mean +/- SEM). No diurnal variation in the concentration was found. Total EGF content was closely correlated with the volume of milk expressed, suggesting a passive transport from the circulation. These observations confirm that a substantial amount of EGF is present in human milk and that EGF concentrations are not affected by duration of gestation, time of day, or duration of lactation.