most effective, followed by MEA, 5 - HT and AET. The average dose-reduction factor for all agents and mouse groups was All this shows that protection 1.35. against life-shortening is qualitatively and quantitatively different from protection against the acute lethal effects and results from a complex interaction of factors depending on strain, sex, drug and dose of radiation. 308. Other experiments on the subject of chemical radioprotection were re- ported by Maisin et al. [M8, M23] and summarized in Maisin et al. (M10, M24]. Balb/e and CS5S7BL mice were given 100-2000 R acute exposures of 250 kVp x rays; causes of death were classified among 12 different groups and analysed for competing risks of death. In the Balb/c strain life-shortening had a linear de- pendence on dose, except perhaps at very high doses. When AET or a mixture of radioprotectors (glutathione, cysteine, AET, MEA and 5-hydroxytryptamine) were administered prior to irradiation with various schedules of administration, they showed a significant protective action against late death. Under the hypothesis of linearity, the dose reduction factor for AET was estimated to be 1.23 + 0.05 and that for the radioprotective mixture 2.1 + 0.2, which values are signifi- cantly smaller than those applying to acute lethality (1.7 and 2.8 respectively). Radiation-induced shortening of life was attributed to specific diseases (thymic lymphoma, myeloid leukaemia, glomerulosclerosis, non-tumorous lesions of the lung). Protection was most effective against thymic lymphoma, but was also dis- cernible for leukaemia and nephrosclerosis. In the C57BL mouse the data, al- though less complete, were essentially similar. 309. Maisin and his collaborators [M24] performed also another experiment where the mice were given fractionated treatments. Using a variety of dif- ferent doses and fractionation intervals they showed essentially that both the irradiated and the irradiated-AET-protected mice die when an accumulated exposure of about 2000 R is reached, whereas mice protected with the abovementioned mixture may sustain exposures up to 4000 R, independently of the size and frequency of the radiation dose fractions, making due allowance for early mortality. 310. Another paper was also reported on the same subject by Philip [P2]. In this case AET (300 mg/kg body weight) or 5 - HT (75 mg/kg) were given i.p. 10, prior to irradiation with 400 R (250 kV x rays) to young Swiss female mice. Single exposures of 100, 200 and 400 R were also given to normal, non-protected