66. Lindop and Rotblat |L1, L2] reported on experiments with SAS/4 inbred mice exposed to single whole-body irradiation (50-780 R, 15 MeV x rays). When the percentage survivors was plotted against age at weekly intervals for each dose group, the life-shortening effect for the pooled sexes fitted a good linear relationship with dose without apparent threshold. The data suggested that life-shortening was the result of a loss of early life and not of a contraction of the time scale: simply from the point of view of survival, the irradiated animals behaved in fact like the non-irradiated controls of older ages. Lindop and Rotblat [L2] established the cause of death of these animals and came to the conclusion that life-shortening was not due to induction of specific diseases but to the forward displacement in time of all causes of death. In this respect radiation could this be considered as an aging factor, although not identical to natural aging, since the relative ages of onset of the various diseases were different in irradiated and control animals. 67. Storer's [S19] data on RF/J mice were limited to a single exposure of 400 R of 250 kVp x rays administered at the age of 90 days. marized in Table 1. They are sum- Storer performed also other experiments [S20] on DBF1/J female mice treated at three months of age with graded exposures (100, 300, 500 R) of 250 kVp x rays. Under these conditions shortening of median survi- val followed a linear non-threshold function of dose. Autopsies performed on large samples of the animals showed that tumour incidence was not increased by radiation exposure, although tumours tended to occur earlier. The time inter- val between irradiation and the occurrence of a significantly increased death rate was inversely related to the size of dose, as though low doses required longer times for the injury to become manifest. On this basis Storer postu- lated that in experiments where animals are sufficiently long-lived (or the latent period is sufficiently short) so that the elevation of the death rate may show over the time interval in which essentially the whole population is dying out, life-shortening will be proportional to radiation dose. But with iow doses or short-lived animals a curvilinear relationship might apply. 68. Upton and collaborators [U7, U9] performed an exhaustive series of ex- periments on RF/Un mice irradiated with various doses and dose rates of 1 and > MeV neutrons, 250 kVp x rays and 605, gamma rays. At high dose-rate (about 10 rad/min or higher) both with x rays and with 1 MeV fast neutrons, the shape of the curves appeared distinctly non-linear (convex upwards). With x rays