55. DNA damage is, however, only the initial step in reactions of this kind, since it is well known that this damage can be repaired. Hart 'H17! dis- cussed the most recent data concerning another complementary working hypothesis. This envisages the aging process as a sequence of events involving the induction of the DNA damage and its subsequent manifestation at the physiolo- gical level. The ability of the system to repair DNA damage and the redundancy of the genetic information for vital functions within the system would be the factors controlling the expression of such damage. Alterations in one or both of these mechanisms would be expected to modify life expectancy. Although interesting, at present both these hypotheses have not been sufficiently formalized and their general applicability has not been extensively tested to warrant more than the present mention in the course of this document. hy 56. Conclusions In summary, although at some stage research on aging was advocated on the ground that radiation might represent a unique tool for the study of senescence [C7] resulting efforts have been on the whole rather unproductive. Data in ani- mals an man lend no support to the view that radiation may cause premature aging or that the carcinogenic effect observed is only part of amore general effect of acceleration of aging [B6]. Attempts to identify a possible life-shortening action with non-specific diffuse changes in tissues, partilularly of the connective and vascular structures have been difficult and are probably inapplicable at low doses and dose-rates. Information about a possible role of the immune system via an increased incidence of auto-immune conditions or a favouring influence on tumour acceleration or induction are few and contradictory. In any case, such mechanisms would not be expected to yield non-specific lifeshortening without changes in the spectrum of old-age diseases. 57. Therefore, in view of the difficulties of defining aging, of the lack of reliable parameters of senescence, of the impossibility to distinguish between specific and non-specific causes of aging and between the action of genetic and ambient factors, of the generally negative conclusions to be drawn from the available data, the Committee decided to limit the present analysis to the only effect of radiation that has convincingly been shown, namely the shortening of life-span. It would in fact be unreasonable under the extremely undefined con- ditions discussed above, to carry out an analysis of the physical and biological