The catalytic activities of recombinant cytochrome P450s expressed in E. coli have been impeded by the absence of endogenous P450 reductase. To solve this problem, we coexpressed P450 reductase with CYP3A4. Membranes from this strain contained 215 pmol P450/mg protein and a reductase activity of 1315 nmol cytochrome c reduced/min per mg. We detected 6ß-hydroxylation of testosterone and oxidation of nifedipine in vivo with turnover numbers of 15.2 and 17.3 min , respectively. These values compare favourably with those obtained using an optimally reconstituted system. Our data demonstrate that a catalytically efficient human P450 system can be generated in E. coli.