Third-generation isotransplants of a C3H mouse mammary tumor have been irradiated with fractionated doses of 250 kV x-rays. All the treatments consisted of five equal doses with the time between each fraction varying from 0 to 5 days. The tumors were irradiated under hypoxic, aerobic, or hyperbaroxic conditions, and response was expressed as the dose required to produce local control in 50% of each treatment group (<tex-math>${\rm TCD}_{50/120\ \text{days}}$</tex-math>). The results have been interpreted in terms of a cell-survival theory and indicate that extensive repair, repopulation, and reoxygenation take place during the course of treatment. Further, by appropriate spacing of fractions, tumor response can be maximized-the optimal spacing being similar for both aerobic and hyperbaroxic treatments.

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