the model's analysis are consistent with available data at comparatively low doses, including the convex upward life-shortening responses. It predicts enhancement of effects after fractionated exposure to 605, gamma rays and an approximately linear response in most cases of acute exposure to low-LET radiation. The model also provides a means of extrapolating between mouse strains or age groups, an extrapolation which can be achieved by changing a single parameter [S48]. D. 1. 35. LIFE-SHORTENING AND AGING Specific and non-specific life-shortening There is considerable discussion in many of the papers reviewed about the specificity or non-specificity of the life-shortening observed in a variety of experimental situations. Semantic considerations as well as impor- tant reasons of substance have complicated this issue. It should first be recognized that speculations about specificity often conceal the lack of good pathological analysis. some specific cause. Life-shortening must be due, if properly assessed, to However, the word specific has been taken to mean that the irradiated animals die earlier than their controls with a spectrum of diseases or causes of death different from the spectrum seen in the non-irradiated controls. 36. Since it is well known that not all diseases are readily induced by ra- diation, to expect that radiation acts non-specifically in shortening the average life of an animal population would be equivalent to reject all radiobiological experience. Recently the discussion has been more reasonably cen- tered on whether or not radiation may produce life-shortening by induction of tumours and how much of the observed shortening could be accounted for by neoplastic diseases. Even though never stated quite clearly, the words spe- cific and non-specific have therefore been taken to indicate neoplastic and non-neoplastic contributions to life-shortening. Under these conditions, the question of specificity of the life-shortening action is quite legitimate and of great practical significance. 37. Mole In discussing the problem of specificity, ICRP Publication 14 ‘I1] and 'M2, M3] point out that in order to detect a general non-specific