Flow cytometry and cell sorting techniques have been used together with repeated measurement in an attempt to define better the radiation survival response of asynchronously dividing Chinese hamster V79-171 cells under aerobic and hypoxic conditions. Although the first two decades of cell inactivation have been examined, particular attention has been given to the low-dose range of a few grays, as used in individual radiation therapy treatments. A single linear-quadratic dose-response function was consistently unable to fit both the low-dose and high-dose data satisfactorily, suggesting a two-component response. Separate fitting of the low-dose and high-dose portions of the response yielded alpha and beta values which differed significantly (P = 0.001 to 0.002). The data are consistent with the hypothesis that the observed substructure simply reflects the presence of subpopulations of sensitive (G1-, G2-phase) and resistant (late S-phase) cells, which are resolved in these measurements. These results may have significance for certain situations in radiation therapy and in biophysical modeling of the radiation response.