There is now evidence that several classes of nitro compounds which have been used as radiosensitizers also function as cytotoxic agents specific for hypoxic cells. The 2-nitroimidazole, Ro-07-0582, (1-(2-nitroimidazol-1-yl)-3-methoxy-2-propanol) is a compound of this type, and its effectiveness as a cytotoxic agent is dependent on drug concentration, contact time and temperature. In vitro, Ro-07-0582 in air at 37 degrees C does not cause loss of cell viability at concentrations up to 2 mM, even when in contact for several days. In contrast, hypoxic cells do not tolerate much lower concentrations of drugs, even if the contact time is only a few hours. When the temperature is raised above 37 degrees C, there is a pronounced increase in the slope of the survival curves; for example, at 41 degrees C (for 1 mM Ro-07-0582, (200 microng/ml), the slope changes by a factor of 2-0 relative to that for 37 degrees C. For cells in air at 41 degrees C, as at 37 degrees C, there is no toxic effect at the concentration of drug tested. In the absence of drug, there is no cytotoxic effect of hyperthermia alone under these conditions. These results are discussed in terms of Arrhenius parameters.