Eicosapentaenoic acid (EPA) is a poor substrate for the fatty acid cyclo-oxygenase but is a good substrate for lipoxygenase enzymes which catalyse the biosynthesis of hydroperoxy-acids, hydroxy-acids and leukotrienes. Recently, we reported that leukotriene B5 (LTB5) was at least 30 times less potent than LTB4 in causing aggregation, chemokinesis and degranulation of polymorphonuclear leukocytes in vitro. In this paper, the effect of oral administration of EPA on LTB4 and LTB5 production by rat leukocytes stimulated with the calcium ionophore, A23187, was assessed. The concentration of LTB was determined by radioimmunoassay and also by reverse-phase high pressure liquid chromatography using PGB3 as internal standard. Supplementation of a normal rat diet with EPA (240 mg/kg per day) for 4 weeks caused a significant increase in the formation of LTB5 and a decrease in the synthesis of LTB4 by stimulated leukocytes. The EPA-rich diet significantly increased the EPA content of leukocyte phospholipids without altering the content of arachidonic acid (AA) or linoleic acid. The ratio of EPA/AA in leukocytes correlated (r = 0.795, P less than 0.001) with the LTB5/LTB4 ratio produced after stimulation of leukocytes. If LTB4 has a chemotactic role during inflammation, the present data suggest that an EPA rich diet could decrease the accumulation of leukocytes at sites of inflammation.