Cultured endothelial cells have been shown to regain their physiological function when replaced in the rabbit eye. Corneas were wiped free of native endothelium and seeded with cultured cells. After an incubation period, full-thickness buttons were cut from these corneas and transplanted into recipient animals. Clear grafts were obtained only when the donor cells were derived from cultures less than a month old. Light and scanning electron microscopy showed the endothelial cells of these grafts to be present as a slightly irregular monolayer on the posterior surface of the cornea. In corneas made edematous by benzalkonium chloride, the clear graft remained surrounded by thick and cloudy host tissue. In those grafts with 3H-thymidine--labeled cells, radioactivity was limited to the host tissue.