Partially synchronous cell populations of a transplantable murine mastocytoma exhibiting a cell cycle time of 8-8.5 h were prepared by density gradient centrifugation and reincubation of slowly sedimenting early interphase cells. The single-dose survival curve of such cultures indicated that x-ray sensitivity was high in early S and in${\rm G}_{2}+{\rm M}$, and low in late S and G1 although these cells have a short G1 period. Survival curves of cells irradiated at different stages showed pronounced changes in extrapolation number n, whereas the slope D0 did not vary markedly. In order to assess the cellular ability to repair sublethal damage, the survival ratio was measured at various times after incubation of early interphase cells by the split-dose technique with a 1-h interval. Marked cyclic changes were observed, indicating that mastocytoma cells repair radiation damage most effectively in the middle and second part of S, and to a lesser extent in G1. These fluctuations in survival ratio were in good correlation with the variations in n. These findings support the view that relative increase in survival after split doses and extrapolation number both are the expression of one single type of cellular repair activity. This is in agreement with the concept originally proposed by Elkind and Sutton that the extrapolation number reflects the extent of cellular recovery from sublethal injury.

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