Mitotically synchronized cultures of HeLa S-3 cells were subjected to the treatment of radiation (400 rad), hyperthermia (43°C), and a combination of both at different phases of the division cycle. The radioresistance was most pronounced in the mid G-1 and late S phases, while thermal resistance was greatest in the early G-1 phase and steadily decreased as cells entered the S phase. Cells in the late S and early G-2 phases were found to be most sensitive to hyperthermia. The sequential treatment of radiation immediately followed by hyperthermia resulted in an enhanced cell killing throughout the cell cycle with a marked synergism occurring in cells in the late S phase. The age-response function of the combined treatment was more similar to that of the thermal age response.

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