with age in a complex manner, irradiated animals being generally more sensitive than controls. Finally, recovery rate tested by split exposures was found to decline sharply with age: the rates estimated in previously-irradiated animals were much lower than those in non-irradiated animals of the same age. From all these data Storer [S19] concluded that the tests applied were in fact measuring the damage inflicted to different cellular systems each of which aged at a different rate, in contrast with the notion of a non-specific life-shortenining action. 268. Lindop and Rotblat [L12] made a systematic study of the age factor giving small single sublethal exposures of MeV x rays to SAS/4 mice. They found that radiation given at 4 weeks of age produced an effect that was linear with dose in the range of 50 ~- 780 R. The life-shortening produced by 100 R was about 4 weeks for mice irradiated at the age of 1 week; it increased to about 6 weeks for 5 weeks-old-animals and then decreased steadily to a minimum of approximately 2 weeks for animals irradiated when 70-90 weeks old. A given reduction in life-time in old animals represents a much greater loss of the life to live than the same reduction produced at a young age. When the effect was expressed as a precentage of the remaining life-span the increase in response at 5 weeks of age was still evident, followed by oscillations of the response between a maximum of 6 per cent and a minimum of 3 per cent reduction of the remaining life-span. 269. In a subsequent paper [L16] life-shortening was studied as a function both of the age and of the oxygenation conditions of the animals. For animals breath- ing air, assuming linearity of response at all ages, the life-shortening effect at 1 day and at 1, 4 8 and 30 weeks of age was found to decrease as a function of age from 7.6 to 2.7 weeks/100 R. Under hypoxic conditions a considerable re- duction of the life-shortening effect was found at all doses, amounting to a factor of three for mice irradiated at 8 and 30 weeks of age. However, when the mice were irradiated at 1 day or at 1 week of age hypoxia changed the linearity of the dose-response relationship to a convex upward curve, such that the protection afforded by hypoxia at low doses was large but at high doses small. authors could not suggest a firm interpretation for such findings. 270. Johnson [J2] set up a simulated experiment where he computed the life- shortening as a function of age at irradiation utilizing parameters and lifefunctions taken from Sacher's [S2] analysis of the LAF1 male mice exposed to fission neutrons and gamma rays in the Greenhouse experiments [U5, Fe]. Irra- The