Recent reports that diene hydroperoxides are formed from polyunsaturated fatty acids by enzymes present in platelets and polymorphonuclear leukocytes have stimulated interest in this class of compounds. Arachidonic acid (5, 8, 11, 14-eicosatetraenoic acid, 20: 4), for example, is converted to 12-(hydroperoxy) eicosatet-raenoic acid (1 2-HPETE) by a platelet enzyme,',* and an enzyme present in leukocytes converts this fatty acid into 5-( ...