The lipoxygenase from reticulocytes oxygenates 15L s -HETE to 8-hydroperoxy-15-hydroxy-5,9,ll,13-eicosatetraenoic acid and 5-hydroperoxy-15-hydroxy-6,8,11,13-eicosatetraenoic acid only in the presence of catalytic concentrations of monohydroperoxy fatty acids. During this reaction the hydroperoxy fatty acids are converted to more polar products including hydroxy fatty acids. From kinetic measurements of 15L s HETE oxygenation it was calculated that 1 mol monohydroperoxy fatty acid is consumed during the oxygenation of about 9 mol 15L s -HETE.