THE OCCURRENCE of cataract following exposure of the lens to ionizing radiations has been demonstrated on numerous occasions, both in man and in experimental animals. Cataract occurs in varying degree, depending upon the quality of the radiation, the total number of roentgens given, and the interval of time during which the exposure is made. Thus, a specific amount of the longer roentgen rays given in a single dose is more cataractogenic than the same r dosage of short rays given in divided doses over a period of several days or weeks. In addition, the cataractous reaction in the lens, which becomes visible only after a variable latent period, depends upon the species and the age of the animal exposed to radiation. These circumstances have resulted in considerable confusion concerning not only the ultimate occurrence of cataract and the relationship to radiation, but likewise the fundamental cause of the lens opacification.