The mortality response of male NMRI mice to whole-body irradiation with fast neutrons of the energy range from 1.7 to 5 MeV was studied and compared with the effects of 250 kV x-rays. Fast neutrons were confirmed to be very effective for the induction of early death. With increasing doses of neutrons survival time decreased more rapidly than with x-irradiation so that, in progressing from midlethal to lethal exposures, mean survival time became markedly lower than after equivalent doses of x-rays. The variance in death time was significantly greater in neutron than in x-ray experiments. This reflects the efficiency of additional killing mechanisms attributed to acute gastrointestinal damage. Mean lethal dose was used to determine the relative biological effectiveness of neutrons. The <tex-math>${\rm LD}_{50(30)}$</tex-math> was 293 rads for neutrons and 479 rads for x-rays. From these data, taking account of the additional gamma-ray component, the RBE of neutrons was calculated to be 1.5.

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