Using reverse phase high performance chromatography with UV-detection, the arachidonic acid cascade in human peripheral blood lymphocytes (PBL) was studied. It was found that PBL oxidized arachidonic acid via the lipoxygenase pathway, 12-hydroxyeicosatetraenoic acid (12-HETE) being the major metabolite of endogenous arachidonic acid. Exogenous arachidonic acid added to human PBL suspensions increased 12-HETE synthesis 5-7 times. In another experimental series the effects of gangliosides (GD3, GM1 and GM3) on lipoxygenase-catalyzed oxidation of arachidonic acid in human lymphocytes were investigated. All the gangliosides tested stimulated PBL to secrete 12-HETE both from endogenous and exogenous arachidonic acid. In most cases the stimulating effect of GD3 was much more apparent that those of GM1 and GM3.