Omega and subterminal hydroxylations of prostaglandins (PGs), leukotriene B4 (LTB4) and some related eicosanoids are catalyzed by the cytochrome P450 (CYP) enzymes belonging to the CYP4A and CYP4F subfamilies. CYP4A4, which is induced in pregnant rabbits, is the only elucidated PGE omega-hydroxylase within the CYP4A subfamily. CYP4F3 is the most tissue specific and most efficient LTB4 omega-hydroxylase, judging from its restricted localization in human polymorphonuclear leukocytes (PMN) and its very low Km value for LTB4. CYP4F2 is widely distributed in human liver and other tissues, and catalyzes omega-hydroxylation of various lipoxygenase-derived eicosanoids as well as LTB4, with relatively comparable and high Km values. CYP4F3B is very similar to CYP4F2 in its tissue localization and its Km value for LTB4. Human seminal vesicle CYP4F8 is the first elucidated hydroxylase with substrate specificity for PG endoperoxides, whereas ram seminal vesicle CYP4F21 is the only elucidated PGE omega-hydroxylase within the CYP4F subfamily [corrected]. Rat CYP4F1, CYP4F4 and CYP4F5, and mouse Cyp4f14 have LTB4 omega-hydroxylase activity. Three additional human, four mouse, and one fish members of the CYP4F subfamily have been identified.