A single layer of epithelial cells exists on the anterior surface of the lens just below the lens capsule. Techniques have been described for making whole-mount preparations of this entire layer of epithelium.1,2The advantage of a whole-mount preparation over sections is that one can view and compare regions of the entire epithelium all in one preparation. This procedure has been used to follow the cytological changes after irradiation.2Also, the mitotic index, which can be determined from these preparations, has proven to be a sensitive criterion of the "normalcy" of the lens maintained in tissue culture.3,4 Tritiated thymidine has been used very effectively in studies of cellular proliferation in various tissues.5Thymidine, which is a precursor to DNA synthesis, has been shown to be incorporated into cell nuclei. It is assumed that cells which incorporate thymidine are undergoing a synthesis of DNA in preparation for