Using a classical methodology of purification consisting of three chromatographic steps (Octyl-Sepharose, DEAE-cellulose, CM-cellulose) we have purified a new cytochrome P-450 from human liver microsomes. It was called cytochrome P-450(9). It has been proven to be different from all precedingly purified human liver microsomal cytochrome P-450 isozymes by its immunological and electrophoretical properties. It does not cross-react with any rat liver cytochrome P-450 and anti-cytochrome P-450(9) does not recognize rat liver microsomes; thus this cytochrome P-450(9) is specific to humans. This cytochrome P-450 isozyme exists in low amounts in human liver microsomes and exhibits an important quantitative polymorphism. In reconstituted system, cytochrome P-450(9) is able to hydroxylate all substrates tested but is not specific of any; its exact role in xenobiotic metabolism in man remains to be elucidated.