A murine ependymoma was given four daily fractions of 500 rads of x-rays and single doses of 2300 and 3000 rads. The effects of the three irradiation schedules on the growth kinetics were compared to the changes occurring to the DNA-synthesizing tumor cells. The rates of tumor regression and recurrence were independent of the schedule used. But the 4 × 500 rads schedule was almost as effective in reducing the number of DNA-synthesizing cells as a single dose of 3000 rads. However, the time interval required before tumors given fractionated irradiation resumed growth was shorter than that needed after a single dose of 2300 rads. These results are discussed.

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