Nitroimidazole and nitrofuran derivatives comprise a large family of [antitumor] compounds, some of which are hypoxic cell specific radiosensitizers in vivo and in vitro. The effects of metronidazole (2-methyl-5-nitroimidazole-1-ethanol) and nitrofurazone (5-nitro-2-furaldehyde semicarbazone) were studied on cell viability in vitro in the presence of air or N2 in the absence of radiation. Exponential phase Chinese hamster ovary cells were placed in suspension culture in complete medium in the presence of air, made hypoxic by flowing N2 (< 0.001% O2) and exposed to various concentrations of these drugs. As a function of time, aliquots were removed and plated to determine cell viability. After 8 h of incubation of Chinese hamster ovary cells in 29 mM metronidazole or 500 .mu.M nitrofurazone, the absolute plating efficiency remains relatively constant (80-40%) in the presence of air. Under hypoxic conditions the plating efficiency of the cells dropped to 1% after 6 h of incubation in 29 mM metronidazole or 500 .mu.M nitrofurazone. This phenomenon of hypoxic cell specific toxicity was dependent on cell type concentration of drug, temperature of indubation and concentration. An increased toxicity of these drugs under hypoxic conditions is indicated.