A total of 6316${\rm B}6{\rm CF}_{1}$ mice were exposed to 60 equal once-weekly doses of 0.85-MeV fission neutrons (0.033 to 0.67 cGy per weekly fraction) or60 Co γ rays (1.67 to 10 cGy per weekly fraction) and were observed until they died. The mean aftersurvival times showed that the dose-response curves for both neutron and γ-ray exposures were indistinguishable from linear over all doses except the highest neutron dose. The relative biological effectiveness (RBE) for neutrons, calculated as the ratio of the initial slopes of the dose-response curves, was about 20 for both males and females. Essentially the same value was obtained by a number of other analyses of the data. Virtually all of the radiation-specific excess mortality could be attributed to tumors; after decrementation of the population for nontumor deaths, the value of the RBE was not significantly changed.

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