The increasing awareness of the harmful effects of ionizing radiation has created an understandable reluctance on the part of many clinicians and investigators toward its administration. In contrast to this hesitation there has been a rapid and progressive expansion of facilities for the medical application of x-ray and radioisotopic techniques. Such expansion and improvement in techniques and equipment, while greatly enhancing the safety and usefulness of these agents in diagnosis, have simultaneously served to promote their utilization. With this increased application, the growing awareness of potential harmful effects has placed an added burden of responsibility for judgment in the use of radiation upon the clinician and investigator. In the 1959 Symposium on Low Level Irradiation sponsored by the American Academy for the Advancement of Science,1E. L. Green has aptly stated what to us seems a reasonable approach to the problem, applicable as well to the somatic radiation effects