radiation-induced senescence; induced by early mortality; or of selective changes in the population of mice or by normal “attrition”. They were instead con- sistent with the hypotheses that with advancing age the sensitivity to the induction of certain diseases decreases, irrespective of the time required for their expression; or, alternatively that radiation given in old age might have a therapeutic effect on some neoplastic growths in these animals. 277. Ainsworth et al. [A7] examined the problem of age-sensitivity in B6CF1 male mice after single doses of 80 rad of fast neutrons or 269 rad of gamma rays. The mice were 115, 194 or 278 days old at the time of exposure. Irre- spective of whether life-shortening was expressed as a per cent reduction of the after-expectation of life or as per cent life-shortening, there was some sparing of effect with age and this was shown to be greater after gamma than after neutron irradiation. The contribution of this change in sensitivity to the sparing effect of fractionation during a long course of treatment would thus be proportionately greater with low- than with high-LET radiation. 278. A very comprehensive experiment is presently in progress at the Los Ala- mos Scientific Laboratory to study the late effects of whole-body gamma-irra-— diation on C57BL/6J and RF/J mice. Genetic background, dose, dose-rate and age at irradiation are the main variables under investigation. For what concerns the effect of age, newborn, 2, 6, and 15 months old animals are being treated. A preliminary report covering the influence of genetic background has already been made available [S22] and the next draft of the present report will include other new data as they will appear. 279. The data available for another mammalian species, the rat, are similar to those just discussed for the mouse and show a dependence of life-shortening on the age of irradiated animals. Jones and Kimeldorf [J3] treated male SpragueDawley rats with about 220 rad of fast neutrons obtained by the Be (p,n) B reaction. They belonged to 5 different age groups of 1, 3, 10, 15 and 21 months. Survival rate and life expectancy were decreased and the age-specific death rate was increased by comparison with sham-irradiated littermate controls. The magnitude fo these effects was inversely related to age at exposure from post-infancy up to middle age (10 months). At even older ages there was no discernible change in life-span with respect to control rats. In the opinion of the authors these data would be compatible with Neary's theory [N1! postu-