man effects with experience on other mammals. Mole considered that in the ab- sence of a satisfactory theory, efforts to define a relationship between daily dose and life-span for survival times of the order of 95 per cent or higher of the control values would hardly be justifiable. On the other hand, it is only in this region of effects that extrapolation could be of any interest. 228. In 1955 Sacher [S1] examined the evidence available on seven animal spe- cies (rabbit, rat, mouse, monkey, dog, burro, guinea-pig) treated with various acute or chronic doses and deduced for each species a cumulant lethality function (see paragraph 116) describing the course of injury according to a given set of reasonable hypotheses. Regarding short-term lethality, the con- clusion was that the most and least sensitive species investigated differed by a factor of about ten in the steady-state or plateau values of their cumulant functions, while intra-species differences were within a factor of four. How- ever, the evidence available for long-term mortality showed considerably less species variation. 229. Blair [B4] and with him the 1958 UNSCEAR report [U1] by plotting together rat and mouse life-span-shortening data after acute irradiation and showing their good agreement on the basis of units of LD 50/3 0? implicitly recognized the existence of some relationship between acute and chronic survival response in these two species and the close similarity of their susceptibility to the longterm lethal action of chronic irradiation. 230. Boche's hypothesis [B11] that a given dose might produce the same pro- portional life-shortening in different species has represented the basis of many attempts to derive life-shortening per unit dose in man from laboratory animal data. The values proposed varied from 1 to 5 days/R [N1]. In Neary's model [N1] if the life-shortening/rem is taken to be the same for man as for the mouse, the absolute life-~shortening for the two species would be calculated at 0.08 day/rem of chronic radiation. Thus, a person accumulating what was at the time a maximum permissible life-time dose of 200 rem would suffer a lifeshortening of 16 days, instead of the figures of up to one year calculated on Boche's [B11] assumptions. 231. On the basis of data obtained in the mouse exposed to acute 600, gamma doses (110 to 1200 rad) Spalding, Johnson and McWilliams [S21] attempted some extrapolation. They found that if a mouse-to-man relationship of 1 day to