Mice were protected from radiation injury by exposure to an atmosphere of 5% oxygen during irradiation, and a dose-modifying factor (DMF) of 2.03 was obtained. A significant difference in the split-dose recovery patterns was noted after conditioning doses under hypoxic and air conditions. This was confirmed when an hypoxic conditioning dose was followed by redetermination of the <tex-math>${\rm LD}_{50(30)}$</tex-math> in air. Comparison of these patterns leads to the conclusion that early repair at the cellular level is diminished after irradiation under hypoxic conditions in the particular cell type responsible for death of the mouse. Multiple exposure data and calculated dose-modifying factors for the split-dose situation indicate that a maximum reduction in the DMF from 2.03 to 1.53 can be attained. This reduction in DMF with multiple exposures has significance for radiation therapy, since it indicates that fractionated radiation therapy may obviate some of the problems encountered when a portion of a tumor is hypoxic.

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