A bacterial cell containing a functional cytochrome P450 monooxygenase system, said cell comprising a genetic construct capable of expressing a cytochrome P450 and a genetic construct capable of expressing, separately from said cytochrome P450, a cytochrome P450 reductase wherein the N-terminus of the cytochrome P450 and the N-terminus of the cytochrome P450 reductase are each adapted to allow functional coupling of said cytochrome P450 and said cytochrome P450 reductase within said cell. A bacterial cell containing a cytochrome P450 comprising a genetic construct encoding, and capable of expressing, said cytochrome P450 wherein the cytochrome P450 comprises an N-terminal portion which directs the cytochrome P450 to a cellular compartment of membrane of the bacterial cell. The bacterial cells are useful as, for example, bioreactors, in drug testing and mutagenicity testing and as a source of cytochrome P450.