Bothtrans-N,N'-bis(2,2,6,6-tetramethyl-1-oxyl-4-piperidinyl)-1, 2-diaminocyclopropane[Ro31-2269] and its cis isomer [Ro 31-2778] selectively sensitized hypoxic Chinese hamster cells, line V-79-753B, to radiation by decreasing both the D0 value and extrapolation number, whereas a related dibasic monoradical Ro 31-2655 decreased D0 alone. Although sensitization was maximal after a 1-hr cell-drug contact time, cells continued to accumulate both Ro 31-2269 and Ro 31-2778 when this contact time was increased up to 3 hr. There was no evidence for competition between either biradical and 2,2,6,6-tetramethyl-4-piperidinol-N-oxyl (TMPN) at equimolar concentration or biradical and 0.82 microM oxygen when cells were equilibrated with the biradicals for 3 hr prior to irradiation in the presence of mixtures of either oxygen and biradical, TMPN and biradical, or TMPN alone. Furthermore, when cells were equilibrated with an equimolar radical concentration of the trans isomer Ro 31-2269 and TMPN for 1 hr prior to irradiation in the presence of the mixture, there was no appreciable effect on sensitization of the slope of the hypoxic cell survival curve, but shoulder modification was reduced. When cells were equilibrated with the trans isomer Ro 21-2269 prior to irradiation in combination with 2.92 microM oxygen, cell survival was similar to that seen for cells irradiated with this concentration of oxygen alone. Examination of the plasma membrane from cells equilibrated with the trans biradical Ro 31-2269 showed that the drug accumulated in the membrane when compared with the concentration found in whole cells. Experiments with the conformationally-unrestricted biradical bis(2,2,6,6-tetramethyl-1-oxy-4-piperidinyl) succinate [Ro 03-6061] showed that when cells were equilibrated with the compound for 1 hr prior to irradiation in hypoxia in the presence of a mixture containing an equimolar radical concentration of TMPN, there was an increase in both the slope and the extrapolation number compared with values for hypoxic cells irradiated in the presence of this biradical alone. Furthermore, when cells which had been equilibrated with Ro 03-6061 were washed free of the drug, there was a residual decrease in both the D0 and extrapolation number of the hypoxic cell survival curve for at least 3 hr after removal of the compound. The results are discussed in terms of a model to account for sensitization by these compounds.