A study has been made of the depigmentation response of resting hair follicle melanoblasts to x-rays. The survival curve for these cells extrapolates to a value of about 6 and the D0 value is about 200 rads. Three days after initiating hair growth the physiological state of the cells has altered and the survival curve shows an increase in the D0 and$D_{{\rm q}}$. This is accompanied by a probable increase in cell number. Split-dose experiments with cells believed to be in G0 show a recovery factor of about 6 and a recovery curve which reaches a plateau at this value after time intervals of 6 hours or more. These results are obtained when the experiments are scored in the second postirradiation growth phase. The survival curve obtained when variable second doses are delivered 8 hours after a priming dose of 500 rads shows the same D0 value as the single dose curve but extrapolates to about 2.7. These data can be taken to indicate that there are not more than two melanoblasts in the resting zigzag hair follicle. Histological data appear to support this view. The relevance of these data to split-dose studies in general is discussed.

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