We used a cell sorter assay to evaluate the efficacy of the hypoxic cell sensitizer etanidazole over 3-4 logs of cell inactivation, with particular attention to the clinically relevant low-dose survival region. Analysis of the radiation responses in a panel of six human tumour cell lines under conditions of hypoxia and hypoxia with etanidazole revealed both a cell-line and radiation dose-dependence in the sensitizing ability of this drug. Fits of the linear-quadratic (LQ) model to the low-dose region of cell survival indicate that sensitization of hypoxic cells by etanidazole results primarily from a modification of the beta parameter. This results in selective sensitization of cell lines in which this parameter contributes significantly to cell kill (i.e. a low alpha/beta ratio) and implies that the efficacy of this drug may be tumour specific. Selective modification of beta also leads to a radiation dose-dependence of the sensitizing enhancement ratio (SER). Analysis of the alpha and beta parameters derived from fist to data at low doses of radiation, suggests that the dose-dependence of this sensitizer, and possibly others including oxygen, is cell-line dependent; cell lines exhibiting a low alpha/beta ratio (i.e. with a large shoulder) exhibit little or no SER dose dependence, while those with a high alpha/beta ratio (i.e. small shoulder) exhibit a reduced SER at low doses as compared to high doses. Furthermore, this analysis suggests that modelling of the low-dose radiation survival data under conditions of hypoxia, can be predictive for both the absolute sensitizing ability of etanidazole, and its dose dependence. Our results also indicate that measurement of the in vitro low-dose radiation survival response in a panel of human cell lines is a more effective assay for evaluating agents like etanidazole than simply high-dose measurements in rodent cell lines which have, in general, demonstrated more congruent survival responses.