SR 4233 (3-amino-1,2,4-benzotriazine 1,4-dioxide) is an anti-tumour agent that has a highly selective toxicity to hypoxic cells. In this study we delineate the role of several different bioreductive enzymes in the metabolism of SR 4233 by two tumour cell lines HT 1080 (human fibrosarcoma) and SCCVII (mouse carcinoma). Enzyme kinetics demonstrates similar KM of HT 1080 and SCCVII cell sonicates and differing Vmax. Among all cofactors tested, NADPH was the most important one in reducing SR 4233 by both tumour cell sonicates. NADH was the second most important cofactor while hypoxanthine and N-methylnicotinamide were less involved in the reduction of SR 4233. Carbon monoxide inhibited the reduction by about 60% suggesting that cytochrome P-450 may play a major role in the reduction of SR 4233 under hypoxia in both SCCVII and HT 1080 cells. DT diaphorase is also involved, particularly in HT 1080 cells, in this drug reduction. The level of functional cytochrome P-450, cytochrome P-450 reductase activity and DT diaphorase activity in both cell lines were assayed. These enzyme levels were all higher in SCCVII cells than in HT 1080 cells. This result correlated the higher Vmax of SR 4233 reduction in SCCVII cells than in HT 1080 cells.