Investigations of the biologic effects of different types of ionizing radiations suggest that quantitative differences in response occur even though identical amounts of energy are absorbed per unit volume of tissue. Since the development of technics for the treatment of cancer and other diseases with radioactive isotopes, information regarding the relative biologic efficiency of the emitted beta particles and x-rays is required to utilize properly the knowledge that has been acquired by experience with x-rays. We, as well as others (9), have been impressed with the striking tolerance of normal tissue for beta irradiation. The dosage that may be given without producing necrosis or other serious reactions seems to be many times greater for beta rays than for 200-kv x-rays. Most of the data in the literature regarding relative biologic efficiency deal with alpha rays, neutrons, gamma rays, and supervoltage x-rays. Among the reports comparing beta rays and x-rays there is a lack of uniformity in results obta...