The extent to which rat thymocytes are capable of recovery from radiation injury as measured by several criteria was investigated. Incorporation of <tex-math>$\text{adenine-}8\text{-}{}^{14}{\rm C}$</tex-math> into nucleic acids was depressed by 100 and 50 R, and there was no evidence of recovery of this process within 30 hours after irradiation. Fractionated irradiation with 2 hours between exposures did not reduce the damage to adenine incorporation produced by the same total dose given as a single exposure. Depression of adenine incorporation into nucleic acids and cell survival decreased exponentially as a function of radiation dose, which also suggests that the potential for recovery of radiation injury is absent in mature thymocytes.

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