• If a growth factor could bind to and stimulate human endothelial healing, corneal disease could be minimized. To this end, primary cultures of feline and human corneal endothelium were tested in receptor binding assays for radiolabeled epidermal growth factor (EGF). Both of these cells bound ten times as much 125I-EGF as did the negative control cell lines. The time course of association of 125I-EGF to cat corneal endothelium was found to be complete after approximately 120 minutes at 22 °C. The 125I-EGF was shown not to dissociate greatly when fresh binding buffer was added to endothelial cultures that had bound the radiolabeled peptide. The pH optimum for binding was determined to be approximately 6.4. The receptor number per cell and the affinity constant for binding were determined to be 40,000 receptors per cell and 1.1 × 109 L/mole, respectively, using a Scatchard plot. Parallel cultures of human fetal corneal endothelium grew in vitro only when the growth medium was supplemented with low concentrations of EGF. These studies provide evidence that EGF is specifically bound to the corneal endothelium.