A simple method for the evaluation of the action of pharmacologic agents on the effects of irradiation is described. The criteria selected (mortality rates produced in total body irradiation and changes in spleen, bone marrow and liver) were easily observable and lent themselves to statistical analysis. By means of this method, the following results were obtained: Desoxycorticosterone acetate markedly decreased the lethal effect of various X-ray doses in mice and prevented the radiation-induced fatty changes in the liver. The chemically closely related steroid pregnenolone failed to show these effects. Vit. B complex showed a definite reduction of the fatty changes in the liver. Both Vit. B-complex and Vit. C failed to produce significant changes of the radiation effects on the spleen. The importance of pharmacologic analysis of radiation effects for the understanding of their mechanism is discussed. The significance of the demonstrated possibility to decrease the mortality rates of ionizing irradiations produced in total body irradiation for the improvement of radiation therapy of widely spread cancers and the possible utilization of these experiences in the treatment of persons accidentally exposed to lethal doses of radiations generated during nuclear fission are indicated.