appearance of bone sarcoma with increasing activities administered was reported in dogs. The same observation was true for the same species in the ex- periments of Burikina [B13], thus confirming that a shortening of the induction period for tumours (giving rise to a shorter life-span of the tumourbearing animals) holds true also in the case of chronic uptake of the nuclide. 133. There is some information regarding bone-seeking alpha-emitters. Ina reanalysis by Mays and Lloyd [M12] of data on single injections of 2395, in CF1 female mice by Finkel and Biskis [F3, F6] a progressive decrease of the average survival from injection to death (for mice living beyond 200 days from injection) was seen at increasing skeletal dose, even when the incidence of bone tumours at doses in excess of 3.1 pCi/kg decreased, rather than increased. At these very high doses there may of course be some question about how much a non-specific life-span-shortening might have contributed to the observed de~ crease of the percentage tumour incidence. 13h. Shortening of the latency interval of tumours induced by tion in rats was also shown by Bensted et al. [B14]. 239 Pu injec- But in the same species chronic administration of 239Pu -— in spite of an increased incidence of osteosarcoma, leukaemia and marrow aplasia - was reported to produce minimal chan- ges in the life-span of the treated animals [B15]. 135. The life-shortening observed in dogs carrying osteosarcomas induced by various injected bone-seeking radionuclides (9py, 228om, 228... 2264, gr) and included into the Utah experiment were calculated by Dougherty and Mays [D5]. The accumulated skeletal doses up to one year before death were com- puted and used as the independent variable. In all tumour-bearing animal groups and for all nuclides tested the log of the time from injection to death was a linear function of the log average skeletal dose. Thus, not only was the average life-span reduced by the appearance of bone tumours (whose incidence is a function of dose) but a further reduction of the life-span was seen in tumour~bearing animals by the earlier appearance of tumours at higher doses. Based on life-span-shortening, the efficiency of the various nuclides relative to 226 Ra taken equal to one were 239py = 6; to 0.24. 2280, = 8; 2282. = 2.53 905, = 0.07 These differences could best be interpreted in relation to the local site of energy absorption and to the type of radiation, the surface-seeking alpha emitters being substantially more effective than the volume-seekers.