1.8 may be derived. Some protective effect is also found with fractionated courses of treatment but not with duration-of-life exposures and low drug levels. The protective action of a single drug may cumulate with the effects of other drugs and with the action of concomitant treatments like anoxia, bone marrow transplantation, antibiotics. Whether the protective effects of the drugs on the life-span operates through a decreased induction of tumours or of other non-specific conditions is not clear. However, the incidence of some dis- eases such as kidney sclerosis (which is responsible at medium-to-high doses for life-span-shortening) may be decreased by the action of radioprotective drugs. 327. Isologous marrow infusion acts essentially on short-term lethality: late survival is not correlated with the size of the marrow inoculum or with the amount of marrow shielded. The only long-term effect that appears to be af- fected by transplantation or shielding of haemopoietic cells is the induction of thymic lymphoma or of myelogenous leukaemia. other findings These data, together with [P3, S35, S36] may be viewed as evidence that marrow exhaustion is not a factor to be considered among the causes of natural aging or of radi- ation-induced life-span-shortening. 328. The only generalization to be gained from the experiments where whole- and partial—body irradiation were compared is that partial-body exposure in the range of medium-to-low doses is less effective (both per unit dose and per integral dose) than whole-body irradiation for induction of life-span-shortening. Experiments where doses of many hundreds of rad or higher are given to sections of the body are clearly unsuitable for studies on the pathogenesis of life- shortening, because under these conditions localized structive lesions to the irradiated organs are decisive for survival or death of the animals. Data are unsuitable for other firm conclusions on the causes of death contributing to the loss of life-time. It appears however that inclusion of the kidneys in the ir- radiation field is a prerequisite for induction or acceleration of nephrosclerosis, a factor that in all strains or rodent tested and at doses of a few hundred rad largely contributes to life-span-shortening. The tumour spectra and the pathogenesis of each tumour type are too variable for any meaningful gene- ralization. In cases where leukaemia contributes substantially to the reduction of life, the lower incidence of this disease resulting from the shielding of the haemopoietic system [K12, K16, I2, C2k] could explain the low efficacy of the partial-body irradiation in respect to life-shortening.