The ability to purify cytochrome P450 from liver microsomes was the result of the collective efforts of a number of laboratories, borrowing from each others’ efforts and observatrons. Early efforts to solubrlize the hemoprotein from microsomes through the use of proteases were not successful, resulting merely in the solubilization of a number of other microsomal proteins. The breakthrough event was the solubrlizatron of rabbit liver microsomal cytochrome P450 using the ionic detergent, deoxycholate (1), allowing the monooxygenase system to be separated into three components, a reductase, a lipid-containing fraction and a cytochrome P450-containing fraction. Diethylammoethyl (DEAE)-chromatography was used in subsequent studies to purify further the phenobarbital-induced hemoprotem fraction (2) (later known as CYP2B4), and eventually polyethylene glycol precipitation was used to fractionate the microsomal proteins (3). Various induced forms of rat cytochrome P450 were likewise detergent-solubrhzed, using sodium cholate, and polyethylene glycol fractionated, and subjected to DEAE-chromatography with the added step of hydroxylapatite chromatography in the presence of nomonic detergent (4). Constitutive forms of cytochrome P450 were also isolated, using variatrons of these procedures, employing gentler nomonic detergents and a substrateliganded affinity column (5, 6). All of the procedures depend heavily upon the ability to selectively separate the proteins based upon differences in their abilities to bind to different column packing materials, Minor forms of cytochrome P450 can be separated by sequential passage of protein mixtures through an anion exchange column, a cation exchange column, an hydroxylapatite or hydrophobic column. Separatrons on molecular exclusion columns are not too