The radiosensitizing effect of postirradiation incubation at 42°C, a nonpermissive temperature, was investigated after exposure of Micrococcus radiodurans to sublethal doses of${}^{60}{\rm Co}\ \gamma \text{-radiation}$. The irradiated cells lost their colony-forming ability with this treatment to an extent that depended on the radiation dose and the time and duration of the postirradiation incubation. Sensitivity to the posttreatment at 42°C is of much longer duration than that observed in the case of chloramphenicol. Since the rejoining of DNA strand scissions was not inhibited at this temperature whereas the cell viability was reduced, the restoration of the sedimentation rate of DNA is apparently not a sufficient criterion for the recovery of cells from radiation damage.

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