I. IntroductionBiological evaluation is as indispensable in study of the therapeutic potentiality of radiations as in that of drugs, hormones, or vitamins. This is particularly true of radiations of short wave length, such as x-rays, which are known to produce permanent and irreversible changes.It is well known that there are two fundamentals upon which the evaluation of any therapeutic agent is based (Sollmann, 4): its local action and its toxicity when applied to the entire animal. Studies of the latter type are also called lethal dose studies. In the case of x-rays, relatively satisfactory data are available concerning the local action on various tissues and in different animal species (Ellinger, 2). No such facts have been established, however, for the toxicity of x-rays in general body irradiation. The literature, it is true, contains scattered references to the lethal effect of whole body irradiation for a variety of animals, but most of these data have been obtained under conditions which preclude ...