160. Survival and leukaemia incidence were studied in RF male mice irradiated with 250 kVp x rays by Upton, Wolff, Furth et al. [U11]. After 150 R given in a single exposure longevity was 15.6 months and it increased to 16.3 and 16.5 months when this same exposure was split into two 75 R fractions and given 2 and 6 days apart, respectively. Similarly, 450 R given in a single treatment or in 3 equal fractions at 2 or 5 days interval changed survival from 10.3 to 10.8 to 11.1 months, respectively. These changes are indeed small and of du- bious significance, in spite of the relatively high number of mice per group. They could be attributed to changes in the incidence of reticular tissue tu- mours which are by far the largest part of the causes of death in this strain, particularly after irradiation. 161. Mole [M1, M20, M21! reported that when 1000 R or x rays were delivered in 10 daily fractions of 100 R the mean survival time was shortened by 10 per cent, with respect to controls. When the same total exposure was delivered in 100 fractions of 10 R each the mean survival time was shortened by 37 per cent. Thus, spreading a given dose over a longer time would apparently increase the amount of damage. The same result was obtained on CBA mice when 750 R were given in a single dose or spread out over several weeks. In this case frac- tionation induced a change in the shape of the age-mortality curve and of the age-specific mortality rates owing to the appearance of more leukaemia deaths after the protracted than after the single exposure. It seems possible there- fore that the important factor in this case might have been a change in the spectrum of the induced diseases rather than the fractionation per se. 162. Cole et al. fractionation. [C18] examined in LAF1 mouse the influence of 250 kVp x-ray The incidence of leukaemia was increased significantly when 690 R were subdivided into 2, 4 or 8 equal fractions separated by 8 weeks, 19 days or 8 days, respectively. Irradiation shortened survival time in all groups, but the largest decrement was seen in mice receiving 8 exposures of 85 R, an effect which would be contrary to expectation if leukaemia had not specifically shortened survival in this case. In contrast, observations on nephrosclerosis showed a decreased incidence of this disease with fractionation from more than 50 per cent in the mice receiving the single exposure to less than 10 per cent in the group receiving eight fractions. Therefore, in these experiments nephrosclerosis was mainly responsible for early death after the single dose, whereas malignancies specifically accounted for more than half of the deaths in the fractionation groups.