Experiments were conducted to determine the <tex-math>${\rm LD}_{50(30)}$</tex-math> of wild-caught cotton rats (Sigmodon hispidus). Twenty-four groups of six animals each, segregated by sex and weight, were exposed to acute whole-body cobalt-60 radiation between December and February in 1962, 1963, and 1964. Each group category was irradiated at six exposure levels between approximately 300 and 1500 R at rates between approximately 19 and 26 R/min. Controls were maintained for each category, and all animals were treated and housed individually. Probit analysis of mortality data yielded an <tex-math>${\rm LD}_{50(30)}$</tex-math> of 1155 R for the combined groups, indicating a relatively high level of "resistance" to radiation damage, or an exceptional recuperative mechanism. No appreciable sexual difference was noted, although young adult animals exhibited slightly more "radio-resistance" than old individuals. A cursory literature review emphasized the restricted application which can legitimately be made of LD50 data obtained under dissimilar conditions. The data now available on high resistance in several wild rodents, compared to relatively low figures previously obtained for mammals in general, preclude wildness per se as a resistance factor. The mechanism of resistance was not identified.

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