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.

Five partially temperature-sensitive mutants were isolated from wild-type Micrococcus radiodurans. Radiosensitivity of the mutants has been determined. The restitution process in the mutants was relatively sensitive to radiation. The delayed DNA repair in the mutants was due to an alteration of the restitution process, but not to change in degradation system. The enhanced cell death in the mutant cell is closely associated with delayed DNA repair.