might relate to auto-immunity and possibly in turn to aging fui5]. There were at the time few results on animals consistent with the hypothesis of a breakdown in the balance of self-tolerance leading to auto-immune conditions, and on the whole the data were thought to be inconclusive in showing positively any such effect. ho, Studies of the late effects of radiation on the immune system of mammals are still relatively few and their results variable according to the test system and radiation doses. In general, intact animals examined individually show little if any effect of age [S45] reticular tissue tumours [U14]. mostly correlated with the presence of When the immune competence of cells from ir- ratiated animals is examined, there is often a significant decrease in cell number, but this appears to be well compensated by an increase in antibody affinity A10]. ted. Other immune functions could, however, be significantly effec- Recent evidence about effects at the whole-body level is rather ambiguous. On the other hand, animals selected for a low antibody response showed a higher incidence of spontaneous malignancies and a shorter life-span than others of the same genetic packground selected for a low response [C30]. On the other hand, total lymphoid irradiation performed on NZB/NZW mice with a high incidence of an auto-immune disease reversed the expression of this condition and thus produced a prolongation of survival [K22]. 50. In comprehensive review of the immunological action of radiation Anderson and Warner [A10] discussed three general hypotheses for the possible induction or acceleration of auto-immune processes. The first one considers that ra- diation could alter tissue constituents to create new auto-antigens or to re- lease previously inaccessible components. The second possibility would act via somatic mutations leading to the emergence of auto-reactive clones. The third would act through the unbalance of natural mechanisms of regulation con- trolling the potential auto-immune expression. Several studies would be in favour of the third mechanism but at the present time few definitive statements are warranted. D1. The first statement might be that the interplay of regulatory mechanisms in the immune system is so complex and variable that radiation effects are hardly predictable and at present cannot be extrapolated with any confidence from one experimental situation to another. Secondly, although immunological mechanisms could actually be operating at high radiation exposures causing