We have observed that when a single linear-quadratic (LQ) function is used to fit the radiation survival response of an asynchronously dividing population of V79 cells, a consistent misfit occurs at low doses. The data can be better described by fitting the low-dose and high-dose ranges separately, and there is evidence of a two-component response. The most obvious explanation is that we may simply be seeing the response of subpopulations of cells of different radiosensitivity: sensitive G1-, G2- and M-phase cells and resistant S-phase cells. The cell sorting assay for cell survival which we have used in these studies may thus be providing sufficient accuracy to resolve these subpopulations, not previously seen in conventional survival measurements. An alternative explanation is that the linear-quadratic function may be inappropriate for accurate description of the radiation survival response at low dose, at least for these cells. To test this hypothesis we have used three other models to fit the data: the single-hit plus multi-target (SHMT) model and the two-parameter repair-misrepair (RMR) model both yielded inferior fits to the asynchronous survival data; the three-parameter RMR model provided an improved fit to the data. The best fit, however, was obtained using a two-population LQ model, which suggested approximately equal numbers of sensitive and resistant cells. When the survival response of tightly synchronized G1/S-phase cells was measured using the cell sorting assay, no substructure was observed. This offers strong support to the hypothesis that the substructure observed in the asynchronous survival response is due to subpopulations of cells of different, cycle-dependent radiosensitivity.