Hypoxic Chinese hamster V-79 cells were examined for light-microscope morphology, progression through the cell cycle, chromosomal aberrations, and viability, after incubation with the 2-nitroimidazole, misonidazole [1-(2-nitro-1-imidazolyl)-3 methoxy-2-propanol]. Cytological examination of cells up to 42 hr after incubation with the drug at 37 degrees indicated that increasing contact time and increasing drug concentrations interfered with cell attachment and progressively slowed cell progression through the cell cycle. Forty-two hr after a 5.5-hr treatment with 5 mM misonidazole, the majority of cells contained heteropyknotic nuclei, whereas less than 3% had progressed into mitosis. Of the few cells that reached mitosis by 42 hr, the level of chromosomal aberrations was 6 times that due to hypoxia alone. However, the majority of metaphases (70%) were unaltered; thus about 2% of the treated cell population passed into mitosis unaltered. After a 5.5-hr incubation with 5 mM misonidazole, 98% of the cells also had lost their ability to produce clones. It is suggested that the cytotoxic effect of this drug on hypoxic cells is that the cytotoxic effect of this drug on hypoxic cells is largely mediated via an interphase cell death, with a minor effect due to chromosome aberrations and cell death from genetic inequality of progeny cells. The ability of misonidazole to kill hypoxic, noncycline cells, which may limit the curability of some tumors with conventional X-rays or chemotherapy agents, makes it of considerable potential interest.