The sensitivity to x-ray and ultraviolet light of a thymine-requiring mutant of Escherichia coli, grown in the presence of 5-bromodeoxyuridine, was determined after different periods of thymine starvation. The effect of previous irradiation of the cells with either x-ray or ultraviolet light on thymineless incubation was also observed. It was found that the bromodeoxyuridine-induced radiosensitization (both to x-rays and to ultraviolet light) could be reversed by previous thymine starvation. The bromodeoxyuridine-substituted cells preirradiated either with x-ray or with ultraviolet light showed a great percentage of cells which were more resistant to the lethal action of thymineless incubation than the unirradiated population.

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