Mitomycin C, a bioreductive alkylating agent with clinical utility against several human tumors, was found to be selectively toxic at a relatively low concentration (1.5 micro M) to EMT6 tumor cells made chronically hypoxic by preincubation in 95% N2-5% CO2 for 4 hr prior to drug exposure. This selective cytotoxicity correlated well with the preferential activation and metabolism of mitomycin C by sonicated cell preparations. The bioactivation of mitomycin C to an alkylating agent by EMT6 and Sarcoma 180 cell sonicates required hypoxic conditions and a reduced nicotinamide adenine dinucleotide phosphate-generating system. Furthermore, the formation of reactive drug metabolites and the disappearance of mitomycin C from the reaction mixture were inhibited by carbon monoxide. The presence of potassium cyanide in the incubation mixture did not affect either the rate of overall metabolism or the rate of formation of reactive metabolites. A high rate of disappearance of mitomycin C from the medium of intact cultures of EMT6 cells was found only in those cultures which were made chronically hypoxic. These data suggest that bioreductive alkylating agents like mitomycin C have the potential to attack selectively the chemotherapeutically resistant hypoxic cell component of solid tumors. Thus, agents capable of bioreductive alkylation should be useful adjuncts to existing therapeutic regimens which are effective against well-oxygenated cells.