1. 225. Inter-species differences Attempts at inter-species comparisons in relation to life-shortening with the objective of an extrapolation between species and eventually of a projection to man, were discussed and proposed repeatedly. An appraoch based on the actuarial Gompertzian analysis of survival paramaters was first put forward as a working hypothesis by Brues and Sacher [B1]. They envisaged that if the linear dependence of the log mortality rate on age (see Figure I) might hold for mammalian species of different life-span, a common origin for the curves and a time-scaling factor might be chosen in such a way that actuarial functions belonging to various species might be made identical. Calculations of this kind using empirical constants obtained on mice and dogs indicated that in the absence of any recovery function the exposure of man to a continuously accumulated tolerance dose in use at that time (0.3 R per week) might decrease the human expectation of life by 10 per cent. Even exposure to background radiation could, in principle, be responsible for about one year of life lost, with respect to a purely ideal situation where background radiation would not be present. 226. On the basis of experiments already reviewed in paragraphs 103-104 and of other experiments by Henshaw [H1], Boche [B11] identified in the parameter alpha (the excess death-rate/week divided by the exposure level in R/week for chronic irradiation experiments) the quantity that would be invariant for each species and might therefore allow inter-species comparison, because it expressed the susceptibility of that species to chronic radiation insult. Based on the value of such a constant Boche attributed about equal sensitivity to the rat, the dog and the mouse, a higher resistance to the rabbit and a higher suscepti- bility to the monkey. 227. In summarizing the effects of long-continued whole-body irradiation of mice, guinea-pigs and rabbits, Lorenz [L7] accepted essentially the radiosusceptibility scale of Boche [B11] and discussed the problem of extrapolating the findings to man. He concluded that man should be considered to be as sen- sitive as the most sensitive animal found experimentally and on this basis proposed that an acceptable whole-body exposure might be 0.1 R per 8-hour day. Such an approach was criticized by Mole [M13] who pointed out that the rela- tivity of the criterion and the fact that extrapolation from animals to man required sufficient evidence of the similarity between man and other animals, to give confidence to the process of filling the gaps in our knowledge of hu-