Reproductive survival of irradiated Chinese hamster fibroblasts is adequately described at all cell ages by an equation of the form$S/S_{0}={\rm exp}(-\alpha D-\beta D^{2})$. The two terms may be related to the likelihood of inactivation by one or two events, respectively. If the inactivating lesion involves damage to the genome, then corrections should be applied to all but G1 phase cell populations to account for genome multiplicity. The appropriate corrections are derived. The corrections render the single event coefficient, a, constant through interphase, in agreement with data for high LET irradiation. The variations in the two parameters with cell age are clearly different, indicating that cyclic changes in radiosensitivity may not be related to any single physiological correlate, and suggesting that there may be mechanistic differences between the two components other than the number of events involved. Synthesized survival data representing an exponential population are well fitted by the same equation, as observed experimentally.

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