A slight modification of Withers' technique has been used to study the survival of mouse skin epithelial cells after using single doses and divided doses of x-rays with intervals up to 21 days between fractions. The general features of Withers' results have been confirmed, but the doses used for the same effect were higher. The single-dose survival curve had a D0 of 135 rads; the dose increment required to compensate for 24 hours interval between fractions was 570 rads. The increase of$D_{2}-D_{1}$ with time after 24 hours leads to an estimate of the doubling time of surviving cells of not less than 36 hours compared with Withers' value of 22 hours. There is evidence of a considerable spread in the proliferation rate of surviving cells. The probability of epithelial regrowth has been shown to be independent of subdivision of the test area, in accordance with the hypothesis that regrowth can take place from a single cell.

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