Forms of human cytochrome P450 (P450 or CYP), such as CYP1A1, CYP1A2, CYP2A6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4, were expressed or co-expressed together with human NADPH-P450 reductase in Escherichia coli. When P450 was expressed alone in E. coli, the expression level of holo-P450 ranged from 310 to 1620 nmol/L of culture. The expression level of holo-P450 decreased by co-expression with the reductase, and the level ranged from 66 to 381 nmol/L of culture. The expression level of the reductase varied depending on the forms of P450 co-expressed, and ranged from 204 to 937 U/L of culture. We assayed the catalytic activity of P450 using E. coli cells disrupted by freeze-thaw. When co-expressed with the reductase, human P450 catalyzed the oxidation of representative substrates at efficient rates. The rates appeared comparable to the reported activities of P450 in a reconstituted system containing purified preparations of P450 and the reductase.