from an initial portion where the effectiveness of the treatment is of the order of 20-30 days of life~shortening per 100 rad to a final slow-rising portion having an effectiveness of 4-5 days per 100 rad. This final slope would apply to all protraction periods, including exposure for the duration-of-life; however, the point of transition between the fast-rising and the slow-rising segments of the curve would be clear at 50 to 100 rad/day but less so at intensities of 10 to 15 rad/day. This flattening of response noted by Mole [M1] and by Sacher [S71] might be related to the establishment of an equilibrium between radiation injury and recovery mechanisms, which would be related to the kinetic modifications of the tissues that are important for long-term survival 191. [S5, L13, Li4, L15]. It may therefore be expected reasonably that the breaking point or tran- sition in the dose-effect relationships would also depend on the kinetic characteristics of the relevant cell lines, which are known to be species-specific. Such an analysis makes it also quite clear that conditions of protracted exposure may not be defined with respect to their life-shortening effectiveness by a single recover constant or residual injury value valid for all conditions of protraction and all animal species. The neutron data would be such [G1] that when the tran- sition from the fast- to the slow-rising portion of the curve is operative, the RBE would change from 2-3 to 5-15, as a result of the change in life-shortening effectiveness. D. RADIATIONS OF DIFFERENT TYPES AND ENERGIES 1. 192. Data The action of different radiations is manifested through a change of effectiveness for the same amount of energy absorbed by the irradiated animal; The spatial distribution of the primary physical events that are responsible for the final biological effect is at the origin of these changes. Radiobio- logically, the effect of densely-ionizing radiations becomes evident through an increased efficiency of the dose, by comparison with a sparsely-ionizing radiation. Under well specified irradiation conditions, when a given effect may be followed for a whole range of doses, the above phenomenon is expressed by the "Relative Biological Effectiveness" (RBE), a factor specifying the efficiency of the test treatment against a low-LET treatment assumed as the standard. Short of these conditions, the higher effectiveness of a densely-ionizing