SUMMARYHomogenates of normal human epidermis synthesized 12‐hydroxyeicosatetraenoic acid (12‐HETE) when incubatedin vitrowith arachidonic acid. The stereoconfigurations of the C‐12 hydroxyl isomers were determined by incubation with potato 5‐lipoxygenase. The synthesized substrate‐specific diHETEs; 5S, 12R and 5S, 12S, were readily separated by high performance liquid chromatograpy. Using this novel methodology, the normal epidermis was found to synthesize predominantly 12‐S‐HETE while, in contrast, psoriatic scale was found to contain 12‐R‐HETE. The 5‐lipoxygenase inhibitors, Merck L‐651, 896, Takeda AA861, and the active metabolite of Syntex lonapalene were found to inhibit 12‐HETE formation in norma