Intestinal crypt survival was studied in mice irradiated in air, 100% normobaric oxygen, 100% hyperbaric oxygen (30 psi), and 6% normobaric oxygen. Exposures from 900-2000 R at the rate of 47 R/min. Intestinal crypt survival was determined by the method of Hagemann, Sigdestad, and Lesher (4). The results demonstrated no difference in crypt survival for mice irradiated in air, normobaric oxygen, or hyperbaric oxygen. The crypt survival of mice was shown to have a D0 from 377 to 380 R and a n from 7.9 to 9.7 for mice irradiated in air, 100% oxygen, or 100% oxygen at 30 psi, while mice irradiated in 6% oxygen had a <tex-math>$D_{0}=529$</tex-math> R with a n=8.1. The DMF (ratio of D0 in 6% oxygen to D0 in oxygen) was found to equal 1.4. The lack of difference in air and oxygen suggests the highly vascularized (oxygenated) gut is maximally sensitive. Animal lethality (<tex-math>${\rm LD}_{50(5)}$</tex-math>) is compared with crypt survival.

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