1- Using a methodology of purification consisting of only one chromatographic step (phenyl-Sepharose) we have purified a cytochrome P-450 isozyme from liver of clofibrate - treated rats. It was called cytochrome P-450 clo. 2- A single polypeptide of mol.wt. 50,000 was visible after sodium dodecyl sulphate polyacrylamide gel electrophoresis. 3- Antiserum raised against the pure enzyme specifically recognized P-450 clo and inhibited more than 90% of the 11- and 12- laurate hydroxylase activities present in clofibrate - treated rats. 4- Clofibrate treatment of the rats resulted in a six fold increase in microsomal cytochrome P-450 clo as judged by immunochemical quantification. This result is in agreement with the increase of laurate hydroxylase activity after treatment by clofibrate.