Cytochromes P-450 plays a vital role in the oxidation of numerous foreign compounds (i.e. drug, carcinogens, insecticide and environmental polutants), was well as diverse endogenous substrates (i.e. Vitamin D, fatty acid, prostaglandin and steroids). Several cytochrome P-450 isozymes have been purified for various sources in the past few years. We have purified eleven cytochromes P-450 from hepatic microsomes from untreated rats and from rats treated with phenobarbital or 3-methylcholanthrene. Three or four of these eleven isozymes appear to be forms that have not been isolated previously. Based on immunochemical properties, these isozymes could be classified in five groups. We also found that eight of the eleven isozymes showed the high catalytic activities for foreign compounds and four of eleven isozymes showed the high catalytic activities for endogenous substrates.