Mitomycin C (MMC) is the prototype bioreductive DNA alkylating agent. To exploit its unique properties and maximize patient responses, different therapeutic approaches have been investigated. Recently, the focus has concentrated on monitoring the levels of the proteins metabolizing the drug and relating these to activity in a regimen referred to as enzyme-directed bioreductive drug development. To be successful, it is important to understand the enzymology of metabolic activation not only in cell lines but also in solid tumour models. A general mechanism of action for MMC has now emerged that is activated regardless of the source of reducing equivalents, comprising three competing pathways that give rise to unique reactive intermediates and different DNA adducts. Partitioning into the pathways is dictated by chemical considerations such as pH and drug concentration. DT-diaphorase stands out in this mechanism, since it is much less effective at metabolizing MMC at neutral pH. At least five different enzymes can catalyse MMC bioreduction in vitro, and as many activities may be present in solid tumours, including a series of novel mitochondrial reductases such as a cytochrome P450 reductase. Competition between reductases for MMC appears to be based solely on protein levels rather than enzyme kinetics. Consequentially, DT-diaphorase can occupy a central role in MMC metabolic activation since it is often highly overexpressed in cancer cells. Although a good correlation has been observed in cell lines between DT-diaphorase expression and aerobic cytotoxicity, this does not hold consistently in vivo for any single bioreductive enzyme, suggesting revision of the enzyme-directed hypothesis as originally formulated.