Corneal transplants differ from skin homo-grafts in that they usually persist indefinitely without indication of a rejection phenomenon. It was demonstrated that corneal connective tissue cells and the endothelium lining its posterior surface, labeled with tritiated thymidine, persist indefinitely when transplanted. The corneal cells of normal rabbits were killed by local freezing and, during the regeneration period, tritiated thymidine was injected repeatedly into the anterior chamber. It was demonstrated by radio-autography that the labeled cells persisted in the intact cornea for 12-13 months. Two months after labeling, similar corneas were transplated to non-labeled host rabbit corneas. Radioautographs of these corneal grafts were prepared of eyes sacrificed at various periods up to 12-13 months. Positive autographs of both the kerato-blasts and endothelial cells were obtained. These experiments demonstrate that corneal transplants retain their donor identity without indication of the graft rejection phenomenon.