163. According to Vogel, Frigerio and Jordan [V11 fractionating 275 rad of neutrons into 3, 4 or 10 separate daily exposures did not result in any significant difference of the mean survival time of CF1 female mice. Kohn and Guttman [K6] studied the effect of 520 R of 250 kVp rays given as a single exposure or in two fractions administered 8 days apart on male and female CAF1 mice. Regarding specifically the effect of fractionation, here again the re- sults were unclear in showing any significant improvement in survival. 64. Vogel and Jordan [V5] examined on CF1 female mice the effect of frac- tionating a weekly dose (300 rad of 600, gamma rays or of 60 rad of fission neutrons) into 1, 3 or 6 equal dose fractions per week. Both radiations were delivered at aprroximately 1 rad/min and the treatments were continued for a total of 13 consecutive weeks, so that the mice were exposed to almost 4000 rad of gamma rays or 780 rad of neutrons. The mean survival times of the gamma-irradiated mice were not significantly different whether they were exposed 1 or 3 times per week. There was some indication that a further dilution of the dose to 6 fractions per week might increase survival but the significance of the data could be questioned. No indication of a sparing ef- fect of fractionation was, however, found in the neutron-irradiated mice. 165. Silini and Metalli [S27] showed in a small experimental series that sur- vival time could be improved by a schedule of fractionation where a condition- ing dose of 150 was followed by 350 R given at seven time intervals between zero and 48 hours. A positive regression of the data amounting to an 8 per cent improvement in survival time was detected. The kinetics of the phenomen followed a pattern reminiscent of the short~term intracellular type of re- covery described in cultured cells by Elkind and collaborators [E44]. 166. Grahn and Sacher [G1] tested the effects of 450 and 750 R of 604, gamma-— radiation delivered as single exposures or in two equal fractions separated by increasing time intervals from 3 hours to 28 days. The regression of survival time versus fractionation interval was negative in 3 our of 4 cases (2 doses x 2 sexes) but none of the regressions were significantly different from zero. The incidence of leukaemia was not consistently modified by fractionation and this disease was not specifically associated with life-shortening, in contrast to what seen in other studies [G4, U11].