The mortality response of goldfish to whole-body irradiation with 2 MeV fast neutrons from a Van de Graaff accelerator was studied and compared with the effects of 200 kVp X-rays. Although the dose ranges leading to equivalent injury were different with the two types of radiation, the dose vs survival time relationship, and the distribution pattern of time of death after irradiation with fast neutrons were similar to those observed after X-irradiation. Mean lethal dose was used to determine the relative biological effectiveness of neutrons. The <tex-math>${\rm LD}_{50(30)}$</tex-math> was 375 rad for neutrons and 1238 rad for X-rays, from which the RBE of neutrons was calculated to be 3.4. The histological changes in hematopoietic tissue (body kidney) and intenstine to 400 or 1000 rad of neutrons were examined. In fish irradiated with 400 or 1000 rad of neutrons, the marked reduction in the number of hematopoietic cells occurred 5-10 postirradiation days. Severe lesions in the intestine were observed only in 1000 rad-irradiated fish at 10 days after irradiation. These results were compared with previous X-irradiation results in fish.

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