Five nitro-substituted imidazoles and three nitro-substituted pyrazoles have been investigated as radiosensitizers for hypoxic mammalian cells cultured and irradiated in vitro. All the compounds sensitize, but the pyrazoles are less effective than the imidazoles. There is no sensitization of cells irradiated in the presence of oxygen. One of the two 2-nitroimidazoles, Ro-07-0582, is much more effective than any of the other compounds and shows a large degree of sensitization in the submillimolar range. At 4 mM concn, the enhancement ratio is about 2.5 which is near the value for the oxygen enhancement ratio for these cells. The cytotoxicity of this sensitizer is low and the sensitizing properties are not diminished by increasing the protein concentration of the medium. The maximum enhancement ratio for this sensitizer is independent of the position of the cells in the mitotic cycle.

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