The cytotoxicity of the nitroimidazole Ro 7-0582 [1-(2-nitro-l-imidazolyl)-3-methoxy-2-propanol] to tumor cells was studied in vivo using 2 transplantable tumors, the EMT6 mammary tumor in BALB/c mice and the MDAH/MCa4 tumor in C3H mice. A high dose of Ro 7-0582 (1 mg/g) was toxic to approximately 90% of the tumor cells, toxicity only occurred when the tumor contained hypoxic cells and toxicity was directed at hypoxic and well-oxygenated cells. There was little or no selective toxicity to irradiated, as opposed to unirradiated, tumor cells. The relative contributions of cytotoxicity and radiosensitization were evaluated in a tumor cure experiment for the MDAH/MCa4 tumor in C3H mice injected with 1 mg/g of Ro 7-0582. Calculations showed that in the absence of cytotoxicity the dose modification factor due to radiosensitization alone would have been 2.02 rather than the value of 2.30 observed. No toxicity was observed at the lower dose of 0.3 mg/g.