Rabbit corneas denuded of their endothelium were coated with bovine corneal endothelial cells (from steers) previously maintained in tissue culture for short (20 generations) or prolonged (200 generations) periods. When grafted back into female rabbits, the corneal buttons remained clear and showed no edema. In contrast, denuded corneas coated with bovine keratocytes and grafted into rabbits became opaque and edematous within 7 days and remained so thereafter. Bovine corneal endothelial cells of the grafted corneas, which had remained clear for over 100 days, proliferated actively when put back into tissue culture. The corneal endothelial cells of the graft were characteristic of the male (XY). The chromosome number of the endothelium of the recipient rabbit was 2n = 44 with sex chromosomes characteristic of the female (XX). Results of the karyotype analysis show that there was no invasion of the corneal button by the recipient endothelium and, conversely, no invasion of the recipient endothelium by the endothelium on the corneal button. These results demonstrate that cultured corneal endothelial cells remain functional in vitro and can replace a damaged or nonfunctional endothelium in vivo.