Recombinant human epidermal growth factor (EGF) was assessed for its capacity to stimulate proliferation of human corneal endothelial cells in vitro in organ culture with 87 human corneas. The EGF in defined serum-free (S-F) media was able to stimulate endothelial cell mitosis (t test P less than 0.01) in matched transected human corneas after a four-day incubation period as judged by histologic studies. Clearly defined endothelial mitotic figures were seen in all stages of cell division throughout the endothelial cell layer. The implication of increasing corneal endothelial cell numbers in donor corneas before transplantation using a human growth factor, potentially available in pure form in unlimited quantities, is discussed.