Using an automated low dose survival assay, the radiosensitizing effectiveness of misonidazole at low radiation dose (0-6 Gy) was measured in cultured mammalian cells. Also measured was its effectiveness at high doses of radiation (0-35 Gy) using the conventional survival assay. In both cases, several concentrations of the drug from 0 to 5 mM were used. The data at low doses were analyzed by a two-parameter mathematical equation with linear and quadratic dose terms, S = e-alpha D-beta D2, which proved to be a good fit to the experimental data at all misonidazole concentrations. It is shown that whereas the coefficient of the quadratic dose term, beta, increases significantly with increasing misonidazole concentration, the drug does not significantly affect the coefficient of the linear term, alpha. The enhancement ratio (ER) of misonidazole is shown to be decreased at lower doses. The clinical implications of this result are discussed.