THO, The guinea-pig, the goat and dog have also been exposed for the dura- tion-of-life and have revealed differences with the mouse related to the sen- sitivity of the various species to haemopoietic death or, more precisely, to the dose-rate level at which the susceptibility of the bone-marrow becomes the main cause of life-shortening. The guinea-pig was consistently more sensitive in two experimental series; goats showed differences between the two sexes and an increased response with respect to the mouse. There was evidence that the bending point of the curve as a function of dose (see Figures X and XI) might occur at dose rates consistently lower than in the mouse, so that the maximum dose that this animal may accumulate in duration-of-life experiments occurs at less than 10 R/day, as compared to the 20 - 4O R/day applying to the mouse. The qualitative response of the dog might be similar to that of the mouse, since the dependencies on the dose of the life-shortening effect change when haemopoietic or non-haemopoietic mechanisms influence survival. Quantitatively, the primary mechanisms of death might be neoplastic non-haemopoietic or possibly degenerative at dose rates below about 3.5 rad/day, while in the mouse the relevant figure would be in the region of 20 rad/day. 150. A large variety of injected, ingested or inhaled radionuclides have been studied for their capacity to induce life-shortening. The largest ex- 45 Ca) or alpha emitters perience refers to bone-seeking beta emitters (2°sr and (226, > 228, a, 228 Th, 238. ; 2395. ; 2M ; 249 Cf and 252 Cf) administered to different animal species under a variety of routes, doses and dosages. Very good correlations were generally found between life-shortening and induction of bone tumours, thus justifying the conclusion that whatever reduction of life is apparent from the experiment it may entirely or almost entirely be explained by tumour induction or acceleration, except at extremely high doses where a specific mechanism of death might produce short-term mortality. This conclu~ sion is very clear-cut and not surprising, since selective partial-body exposure is the mechanism operating in case of internal irradiation and, under these conditions, non-specific damage to the whole body cannot be expected. The data are therefore not strictly comparable to those from whole-body irradiation and the above conclusions carry little weight in respect to the problem of life-shortening specificity.