and fast-neutron experiments could not be attributed with certainty to dif- ferences in body size (and therefore absorbed doses), to hormonal unbalance resulting from damage to the ovaries or to differences in the spontaneous or induced tumour incidence. 253. Holland and Mitchell [H10] studied life-shortening (300 R of 300 kVp x rays) on four inbred male and female mouse strains (C3H, Balb/c, RIM, C5TBL/6), on two hybrid strains (C3CF1 and B6ORFMF1) and a four-way cross between both F, hybrids, C3CB6RFM. 5-6 weeks of age. At the time of irradiation the mice were In all strains and crosses a significant life-shortening effect was observed. Females were about 1.5 times more sensitive than males of the same strain and these data suggested a general and constant effect of sex under all conditions. Unfortunately, the data on the various modes of death are not yet available for a more complete interpretation of these observations. A higher sensitivity of RFM female mice accounted for by a rapid rise of life-shortening over the dose range of 0 - 50 rad was also reported by Storer et al. eo, [Shh]. In experiments by Moskalev, Petrovich and Streltsova [M22] performed on rats treated with fast neutrons (8.5 - 510 rad) or 500 MeV protons (28 1008 rad) observations on life-shortening were also reported. The average life expectancy of rats irradiated with the fast neutron beam did not depend on the animal's sex. The data by Rust et al. [R2] on the guinea-pig showed that the females were more responsive to life-shortening by chronic gamma-ray exposure. This differential effect could not be correlated with the induction of tumours of the female generative organs. 2. 255. Body weight Body weight as a variable affecting long-term animal survival was inves-— tigated by Holland and Mitchell [H10]. Within the same sex, there were substan- tial differences in sensitivity among the strains tested and these differences were highly correlated with body weight at 6 weeks of age, the heavier animals being less resistant to radiation i.e. showing a higher degree of life-shortening. Previous analysis of body weight versus radiation resistance based on early effects and on mature animal had given opposite results [Q1, G12, R5] in that, within the same strain, adult animals with a higher body weight were shown to be more resistant. However, in adult animals body weight is thought