The problem of the mechanism by which radio-resistance of a tumor can be acquired is examined. Three hypotheses to explain the phenomenon are commonly proposed; namely, (a) radiation selection of spontaneous mutant resistant cells in the tumor population, (b) radiation-induced mutation in a tumor cell to resistance, followed by selection, and (c) a complex of radiation effects on the host that interact with the tumor to give the appearance of resistance to the tumor.

An experiment with mouse ascites tumor, specifically designed to maximize the likelihood of producing a true, or genetically, radioresistant tumor line is described; radioresistance was not produced.

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