extensive tissue damage, their possible relevance at the low doses of interest for radiation protection can only be viewed at the present state of knowledge with great reservations. 52. The Committee has reviewed in its 1977 report [U14] the role of the im- mune system in the pathogenesis of radiation-induced tumours. The conclusions pointed to a secondary role of immune reactions in the development of neoplastic conditions, particularly at low doses and dose-rates. No new infor- mation has appeared in the mean time that might change this general proposi- tion. Actually, a most recent reveiw of the subject [S46] confirms the above conclusion and therefore supports indirectly the view that, whatever the role of tumours in radiation-induced life-span-shortening, there is as yet no clear evidence that it might be mediated through immunological mechanisms. 53. There are other hypotheses of aging that have been considered either alone or in conjunction with radiation and for which some experimental evidence has been claimed. The older theories have been discussed by Walburg [W1] and their applicability to a possible effect of premature aging criti- cized since mortality data and causes of death in irradiated animals point to a life-shortening action related essentially to tumour induction. Also, ac- cording to this analysis, exposure of mammals to life-shortening doses of radiation almost uniformly fails to accelerate lesions characteristic of senescence. Su. dered. Recently other hypotheses related to molecular changes have been consi- Cutler [C31] reviewed the concept of primary aging processes. With this term are covered causes which could underlie many different specific disease processes at the organismic level and many age-related losses of function resulting in a progressive decline of general health. At the molecular level, cross-linkage between biologically important molecules by various agents (free radicals and their derivatives, aldehydes) may be postulated to be at the origin of natural senescence and of possible radiation-induced changes. This hypothesis has received little experimental support when applied to cellular and extra-cellular constituents such as collagen, age pigments, etc. It could, however, be more attractive when applied to information transfer mole- cules among which primarily DNA or chromatine. For these molecules a more systematic approach is advocated, on the basis of preliminary data on radia- tion and natural age-induced changes in chromatine.