A polypeptide hormone has been isolated from human urine, human epidermal growth factor. It was assayed by its ability to compete with 125I-labeled mouse-derived epidermal growth factor in binding to human foreskin fibroblasts. The biological effects of the human polypeptide are similar to those previously described for the mouse hormone. These include the stimulation of the growth in vitro of human foreskin fibroblasts and corneal epithelial cells in organ culture, and the in vivo induction of precocious eyelid opening in the newborn mouse. The amino acid compositions of the two polypeptides differ, although certain similarities are present. The estimated molecular weight of the human polypeptide, 5300-5500, is slightly lower than that of the mouse hormone. Both polypeptides apparently compete for the same site on the cell membrane; and antibodies to the mouse polypeptide crossreact to some extent with the human hormone.