In an earlier study using cell sorting techniques to define the radiation survival response of asynchronous Chinese hamster V79-171 cells more accurately, we found evidence of substructure in the response at low dose. In the present work we have attempted to show that this substructure arises from the subpopulations of sensitive ( G1, G2 phase) and resistant (late S phase) cells which are present in asynchronously dividing cultures but which are not resolved by conventional survival assays. Partially synchronized cells were produced by exposure to 1 mM hydroxyurea for 12 h and were harvested 15 min later, yielding a population of viable cells at or just beyond the G1/ S-phase boundary. Parallel experiments were carried out with asynchronous cells. The average of repeated measurements of the radiation survival response of asynchronous cells again showed a significant difference (P = 0.002 to 0.009) between the α and β values evaluated from the data for the low-dose range, 0-2.8 Gy, and the high-dose range, 2.8-14 Gy. For G1/ S-phase cells, however, there was no significant difference between the values of α and β for the low-dose and high-dose ranges (P > 0.5). The results thus support the hypothesis that the observed substructure in the asynchronous response is due to resolution of subpopulations of different radiosensitivities, and they illustrate the advantage of the cell sorter assay for accurate measurements of cell survival, particularly at low dose.

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