Expts. have been carried out with CF1, ABC, A, and C58 mice to determine both the early and late effects of periodic and single exposures to fast neutrons, slow neutrons, [gamma] rays and [beta] rays. In general, the late effects resulting from exposure to penetrating radiations, irrespective of methods of exposure or type of radiation, consisted of generalized atrophy and neoplasia in the hemopoietic organs. Both effects were attended by loss of wt., a modified blood picture, a changed histology and pathology, some increase of tumor incidence in organs other than hemopoietic, and a shortened life span[long dash]all of which served as criteria of radiation effects. Generalized atrophy, in the case of small daily treatments, appeared to result from the inroads of subliminal damage which in time added up to exhaustion of vital reserves. Neoplasia may or may not result directly from the atrophic condition. By utilizing the various criteria of effect, different thresholds of damage were found. Survival time, which furnished one of the most sensitive responses, showed effects following daily exposures in the range of O.ln of fast neutrons and lr of [gamma] rays. Threshold responses in the peripheral blood were at least a factor of 10 less sensitive than threshold survival responses in CF1 mice. No significant sex or strain differences were rioted. The r/n ratio of [gamma] rays to fast neutrons, for the different effects, varied roughly from 8-1, to 35-1. The degree of acute damage (measured by survival time after single massive dose exposures) varied not only with dose but also with intensity of irradiation; a 10-fold increase in exposure time for [gamma] ray treatments reduced the effects observed for a given dose to about 70%. Evidence indicates that the degree of biologic effect varies not only with dose but also with the density of ionization produced. The incidence of lymphoma following exposure to penetrating radiations was raised from 15% in the controls to more than 00% in groups receiving single doses of 500 and 700 r of [gamma] rays. Lung tumor incidence was increased only slightly, if any, by the treatments given. This was even true in CF1 and A mice, both strains of which have a high spontaneous lung tumor incidence and a demonstrated ability to respond to other carcinogenic regions. Following doses of 4,000 to 5,000 rep of [beta] rays, animals that ordinarily have no skin lesions showed skin carcinomas including nearly every type, some animals having 50 to 100 loci. It is significant that hemopoietic tumors were obtained with penetrating radiations and skin tumors with surface absorbed radiations.