The proposed research on the effects of ionizing radiation and hyperthermia on Chinese hamster cells in culture involves three general areas of investigation. The first objective is to obtain quantitative information on effects of hyperthermia either alone or in combination with x-irradiation when the treatments are applied during different phases of the cell cycle. Specifically, the effects of hyperthermic treatments, varying in temperatures and duration, will be studied in terms of changes in lethality associated with variations in the length of time hyperthermic treatments are applied relative to the period of x-irradiation. The second objective is to determine the target responsible for the x-ray-induced delay in cell cycle progression occurring during the G2 phase of the cell cycle. Specific information is desired concerning the ultrastructural and biochemical events occurring normally during G2 and associated with the x-ray-induced G2 delay. The third objective is to obtain quantitative information on the x-ray induction of mutant or variant cells. Specifically, information is desired concerning: the relationship between any variation in radiosensitivity during the cycle as measured by mutant frequency with variation in radiosensitivity measured by chromosomal aberrations or cell killing; dose response curves for mutant frequencies; and any possible effects of hyperthermia either alone or in combination with x-irradiation on the induction of mutant cells.