X-irradiation of Chinese hamster cells at temperatures above 37°C results in enhanced killing response. The magnitude of this thermal effect increases with increasing temperature and varies inversely with dose rate during the exposure of the cells to the combined effects of elevated temperature and ionizing radiation. Postirradiation incubation at an elevated temperature is also effective in enhancing the response but not preirradiation hyperthermia. Split-dose experiments demonstrate that hyperthermia also inhibits the repair of sublethal damage for temperatures up to ∼41°C. Above 41°C, lethal damage expression is enhanced as well. Fluctuations in the age-response structure of cells x-irradiated at 42°C are reduced, a result consistent with a reduced capacity for sublethal damage when cells are hyperthermic during irradiation.

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