200. Sixteen-weeks-old female CF1 mice received 100 rad/week of fission neu- trons for 4 weeks at dose-rates of 1, 3, 6 and 35 rad/min; 6045 gamma rays at the same dose-rates [V3, Vij. or 200 rad/week of In all groups thymic lymphoma was the main cause of death and its incidence was not significantly affected by the dose-rate. On the basis of mean survival time or of mortality curves, life- time was reduced by about 65 per cent by neutrons and by about 50 per cent by gamma rays. The neutron efficiency gamma by more than a factor of 2. was therefore higher than that of the The mean survival times of both neutron- and gamma-irradiated animals were significantly shorter with 35 than with 1 rad/min. 201. Vogel and Jordan [V5, V6] compared the lethality of fission neutrons pro- duced by a CP-5 reactor to that of 605, gamma rays, both radiations being delivered at about 1 rad/min. CF1 female mice were exposed according to a complex pattern of fractionation in which 13 weekly doses of 300 rad of gamma rays were delivered into 1, 3 or 6 equal fractions per week. Since the arbitrary RBE va- lue chosen was 5, the corresponding total weekly dose of neutrons was 60 rad, delivered into 1, 3 or 6 fractions per week. The data showed that the postu- lated RBE of 5 was too high. 202. Upton, Randolph and Darden [U10! reported that with fast neutrons the life expectancy of RF female mice was shortened by 0.8 days/rad, irrespective of dose-rate, whereas the life-shortening efficiency of the gamma rays decreased from about 0.3 days/rad at 7 rad/min to about 0.15 days/rad at 5 rad/day. The RBE increased therefore with decreasing dose-rate from about 2.7 to 5.4. 203. In the experiments of Upton, Randolph and Conklin [U7] variable dose- rates of x and gamma rays (80 - 1 rad/day) and of fast neutrons (11 to 0.004 rad/day) were administered to RF mice. 100 rad/min of both radiations. There were also groups treated at about Gamma rays at the low dose-rates were invari- ably and consistently less effective than at the high dose-rates. the contrary, showed less dependence on the intensity. Neutrons, on The RBE evaluated on the basis of the average life-shortening effect (days/rad) was about 3 at high dose-rates and it increased to about 8 for terminated chronic irradiation. When exposure continued until mortality reached about 50 per cent of the mice, a further increase of the effectiveness to about 14 took place. Without knowledge of the dose-rate-RBE relationships it was impossible to foresee whether higher RBE's might be found at even lower dose-rates. predict any trend at different neutron energies. Similarly, it was impossible to However, since 5-MeV neutrons