The rates of incorporation of$[2\text{-}^{14}{\rm C}]\text{thymidine}$ into growting attached cells, and stationary suspensions of cells, were compared after electron irradiation in air. Incorporation into both the pool and the cellular DNA was reduced to a greater extent in the stationary cells than in the plated cells. Depression in the suspensions was accompanied by a comparative increase in pool size at the lower doses responsible for the sensitive component of the dose-effect curve for DNA synthesis. Following irradiation, prolonged exposure to thymidine led to a greater depression of its incorporation. When thymidine concentrations were reduced (⋜ 10 μM) an enhanced effect of radiation was observed which could not be accounted for by an observed equivalent expansion in the endogenous-thymidine pool. The hypothesis is advanced that radiation damage to the cell membrane was responsible for this effect, and that such a lesion could lead to the postirradiation differences observed in the attached and the suspended cells.

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