The superfamily of cytochrome P450s encompasses a vast arena of biologically important reactions. The ever-increasing numbers of P450s and the diversity of their enzymatic properties dictate the need to develop new approaches for studying their chemical, physical and catalytic properties. The heterologous expression of P450s in various cell systems (e.g., COS cells, yeast, E. coli, etc.) now provides a means of producing recombinant proteins for such studies. The example is presented of the expression of P450(17)alpha in COS cells and the use of this technique for the comparison of the enzymatic properties of the rat, bovine and human enzymes. Further, studies are described whereby cotransfection results in the simultaneous expression of more than one P450 permitting the construction of 'designer membranes' for assessing protein-protein interactions and the reconstruction of complex pathways of metabolism. Recent advances with genetically engineered systems point to the power of the transfection technique for the study of structure-function relationships with this class of important hemoproteins.