Intestinal crypt survival was studied using the method of Hagemann, Sigdestad and Lesher (8) in mice irradiated with 250 kVcp x-rays, cobalt-60 and fission neutrons. The D0's for the three treatment modalities were 377, 375 and 103 rads with extrapolation numbers of 6.2, 9.2 and 3.1, respectively. The RBE (ratio of D0's relative to x-rays) was 3.6 and 1.0 for neutrons and cobalt-60, respectively. The <tex-math>${\rm LD}_{50(5)}$</tex-math> values were found to be 1081 (945-1247), 1365 (1279-1456) and 252 (234-273) rads. The RBE [ratio of <tex-math>${\rm LD}_{50(5)}$</tex-math> relative to x-rays] values were found to be 4.3 and 0.8 for neutrons and cobalt-60 irradiation. Intestinal crypt survival varied from 25 to 34% of control at all of the <tex-math>${\rm LD}_{50(5)}$</tex-math> values. Total and per crypt cellularity was determined at 3 days postirradiation for each modality. The maximum proliferative response, measured at this time, occurred at or near the <tex-math>${\rm LD}_{50(5)}$</tex-math> dose for each type of radiation.

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