The present investigation was carried out to determine if thymocyte radiation injury leading to interphase death is repaired in vivo. For this purpose either thymus DNA (an index of cell number) or thymus cell number were measured following single and fractionated whole-body x-irradiation. Three days after single exposures, thymus DNA decrease was determined to be dose dependent within the range of 0-175 R. The principal mechanism for this rapid decrease in DNA has previously been shown to be due to interphase death. Two 75-R exposures separated by 4 hours reduced the number of thymus cells and thymus DNA to the same extent as a single 150-R exposure. Additional experiments showed no differences in DNA content of the gland with intervals between exposures of 2 and 7 hours. Fractionation intervals of 10, 15, and 20 hours resulted in increased DNA as compared to single exposure. Measurement of the mitotic index in the thymus as a function of time after a single exposure of 75 R suggests that increased DNA in the gland after fractionation intervals greater than 7 hours is the result of cell division completed between fractions. Therefore, the method cannot be used to study recovery when fractionation intervals exceed 7 hours. Within this limitation the study shows no detectable recovery of radiation injury leading to interphase death in vivo.

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