cDNA of a novel human cytochrome P450 was cloned from human liver by reverse transcription-polymerase chain reaction and designated CYP4F12. The open reading frame coded for 524 amino acids, and the sequence could be aligned with 78-83% amino acid identity to the four human CYP4F enzymes (CYP4F2, CYP4F3, CYP4F8 and CYP4F11). Northern blot analysis suggested three major transcripts of CYP4F12, which were detected in liver, kidney, colon, small intestine and heart. The CYP4F12 gene contained 13 exons and was located at chromosome 19p13.1. CYP4F12, expressed in yeast, oxidized arachidonic acid to 18-hydroxyarachidonic acid, and the omega -side chain of two stable prostaglandin (PG) H-2 analogs (11.9 epoxymethano-PGH(2) and 9,11-diazo-15- deoxy-PGH(2)). CYP4F12 oxidized the omega -side chain of leukotriene B-4, PGE(2), PGF(2 alpha), PGH(2), and 9,11-epoxymethano-PGH(2) poorly. Several CYP4F enzymes are important omega1- and omega2-hydroxylases of eicosanoids. The physiological function of CYP4F12 merits further investigation.