The effect of the incorporation of 5-bromodeoxyuridine (BUdR) in the DNA of T cells on radiosensitivity after irradiation with X-rays and with ultraviolet light has been studied. Radiation survival decreased both after X-ray and ultraviolet light. BUdR incorporation into T cells causes a lengthening of the cell cycle which could be attributed to selective increases of the G1 and S phases. This change in age distribution of the cells might slightly affect the radiosensitivity based on survival. The incorporation of BUdR during one generation cycle before X-irradiation caused an increase in the induction of breaks in the 5-bromouracil (BU)-containing strand over those found in the opposite strand of DNA from the same cells. This increased break induction was found only after X-irradiation and treatment with high concentrations of BUdR. The induction of breaks in both strands was always linear with dose. Irradiation with ultraviolet light produced practically no induction of breaks in normal T cells. After exposure of the cells to a relatively low concentration of BUdR during one generation cycle, irradiation with ultraviolet light produced a large number of breaks in the BU-containing strand but no increase in the opposite non-BU-containing strand of DNA. Break induction in the BU-containing DNA strand was found to be nonlinear with dose. No correlation could be found between the increased radiation-induced mortality and the increased number of primary-induced single-strand breaks in the BU-containing DNA strand.

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