We succeeded in cloning 12-lipoxygenase cDNA from human platelets. A cDNA library of human platelets was prepared and screened using the 15-lipoxygenase cDNA as a probe to obtain a cDNA encoding a protein consisting of 662 amino acids. It was expressed in COS cells and its activity was confirmed. It was shown that this enzyme is induced when HEL cells are stimulated with TPA.