Human and rabbit reticulocyte 15-lipoxygenase (15-lipoxygenase-1) and the leukocytetype 12-lipoxygenases (12/15-lipoxygenases) of pig, beef, mouse and rat constitute a particular subfamily of mammalian lipoxygenases (reticulocytetype lipoxygenases) with unique properties and functions. They catalyze enzymatic lipid peroxidation in complex biological structures via direct dioxygenation of phospholipids and cholesterol esters of biomembranes and plasma lipoproteins. Moreover, they are a source of free radicals initiating nonenzymatic lipid peroxidation and other oxidative processes. Expression and activity of reticulocytetype lipoxygenases are highly regulated. Moreover, the susceptibility of intracellular membranes toward these lipoxygenases is controlled and may be increased together with lipoxygenase activity under conditions of oxidative stress. Thus, oxidative stress may favor a concerted package of lipoxygenase mediated enzymatic and nonenzymatic lipid peroxidation and cooxidative processes. Reaction of reticulocytetype lipoxygenases with lowdensity lipoprotein renders the latter atherogenic and appears to be involved in the formation of atherosclerotic lesions.