The role of microsomal cytochrome b5 (Cyb5) in defining the rate of drug metabolism and disposition has been intensely debated for several decades. Recently we described mouse models involving the hepatic or global deletion of Cyb5, demonstrating its central role in in vivo drug disposition. We have now used the cytochrome b5 complete null (BCN) model to determine the role of Cyb5 in the metabolism of ten pharmaceuticals metabolised by a range of cytochrome P450s, including five anti-cancer drugs, in vivo and in vitro. The extent to which metabolism was significantly affected by the absence of Cyb5 was substrate-dependent, with AUC increased (75-245%), and clearance decreased (35-72%), for phenacetin, metoprolol and chlorzoxazone. Tolbutamide disposition was not significantly altered by Cyb5 deletion, while for midazolam clearance was decreased by 66%. The absence of Cyb5 had no effect on gefitinib and paclitaxel disposition, while significant changes in the in vivo pharmacokinetics of cyclophosphamide were measured (Cmax and terminal half-life increased 55% and 40%, respectively), tamoxifen (AUClast and Cmax increased 370% and 233%, respectively) and anastrozole (AUC and terminal half-life increased 125% and 62%, respectively; clearance down 80%). These data from provide strong evidence that both hepatic and extra-hepatic Cyb5 levels are an important determinant of in vivo drug disposition catalysed by a range of cytochrome P450s, including currently-prescribed anti-cancer agents, and that individuality in Cyb5 expression could be a significant determinant in rates of drug disposition in man.