in fact proposed, of which the majority were obtained from gamma-ray chronic irradiation data (see Table 3). Most of these data agree in showing that the rat, the dog and the mouse are about equally sensitive, while (on the basis of very scanty data) the rabbit would appear to be less susceptible and the monkey, the goat and the guinea-pig perhaps more sensitive. As to the man, its sus- ceptibility is reported in one case to be higher (perhaps by a factor of two) and in another case to be similar to that of dog and the mouse. It does not appear from the data that the differences between the various mammalian species testedis very large and the range within which all species might be included could possibly be a factor of two in both directions (taking the mouse and the dog to be in the middle of an ideal radiosusceptibility scale) or about a factor of five over the whole range of radiosensitivity of the species tested. 283. Intra-species or inter-strain variability has also been studied in the mouse, the species where different genetically-homogeneous strains are more easily available. When adequately looked for, differences between various strains (for the same sex) were easily observed. No formal genetic analysis of the radiosensitivity parameters has been attempted and most of the data refer to irradiation for’ the duration-of-life. In general the amount of life-shor- tening is correlated with the control mean survival time, in the sense that the proportion of life lost per unit dose is similar for the various strains having different life-spans. Life-shortening is also correlated with the expression of the pathological characteristics of the strains, since animals prone to the development of leukaemia and of ovarian tumours (the cases more thouroughly an- alyzed) show a greater amount of life-shortening per unit dose. It is possible that the differences in response of the various strains may also reflect the maturation rate of the genotypes irradiated, as some data on the correlation of the response with the weight of maturing animals would suggest. When allowance is made for all these variables the mouse appears to respond according to a baSic parameter, whereby for acute single doses the number of days lost per unit exposure equals 0.28 days/R. On this basic dose-response relationship all the factors mentioned above (in addition to other factors for sex and age) would superimpose to give the compounded final value of effect applying to each particular situation. 28h. In few of the data reported either no difference or small differences in sensitivity to life-shortening were reported between male and female animals: