mice other non-neoplastic causes were observed in the majority of cases. An increases and advanced incidence of specific diseases, mainly thymic lymphoma, was at the origin of the radiation-induced life-shortening in the low-to-medium range of exposures. For higher doses in excess of the LD... life-shortening was D0 instead characteristically associated with glomerulosclerosis. 78. Very extensive data on RFM and Balb/c mice were recently Ullrich and Storer [U8] and Storer et al. [Shh]. presented by The effects of dose, dose-rate and radiation quality on life-shortening and carcinogenesis were examined. 13% os gamma rays at 40-45 rad/min and 8.3 rad/day (10 to 400 rad total doses) and fission neutrons at 5-25 rad/min or 1 rad/day (5 to 188 rad total doses) were used. Dose-effect relationships for life-shortening at dose-rates will be examined here. In the RFM females a dose-squared or linear-dose-squared model could describe the data adequately between zero and 50 rad, with the dose-squared component predominating after about 4 rad. The curve for RFM male animals was thought to be linear. High dose-rate neutron curves in both RFM and Balb/e females were linear in the range of zero to 47 rad, with an ensuing decrease of effectiveness giving rise to an upward convex trend up to 200 rad. No specific discussion is given of the contribution of particular diseases to life-shortening. 79. Metalli et al. [M9] irradiated hybrid male mice of the (CSTBLxC3H)F, strain (100-days old, 250 kVp x rays, 100 to 700 rad). A dose of 900 rad with bone marrow infusion from isogeneic donors or with shielding of one leg in order to overcome the early effects of radiation on survival was also used. These procedures did not appear to alter appreciably the long-term survival of the animals. The mean after-survival of the mice as a function of dose could be reasonably fitted by a linear function. These animals have a spontaneous incidence of about 55-60 per cent of reticulum cell sarcoma. The incidence of this disease was still quite high at 400 rad but fell gradually at higher doses to about 5 per cent at 900 rad. On the contrary, the incidence of glomerulo- sclerosis (which is very low in normal animals) increased to about 70 per cent after 900 rad. Since life-span-shortening versus dose could be fitted by a linear regression in spite of such profound changes in the spectrum of induced diseases should probably be regarded as a fortuitous event. The data are in no way reconcilable with any theory postulating a non-specific aging effect. 80. Preliminary data have also been reported about a very extensive study designed by Spalding et al. [S22] to investigate in the same experimental