Docosahexaenoic acid (DHA) or eicosapentaenoic acid (EPA) was facilely incorporated into phospholipids of mouse peritoneal macrophages following incubation with pure fatty acids complexed to bovine serum albumin. Following stimulation with calcium ionophore A23187, the DHA-enriched cells synthesized significantly smaller amounts of leukotriene C 4 and leukotriene B 4 compared to control or EPA-enriched cells. The EPA-enriched cells synthesized lower amounts of leukotriene C 4 and leukotriene B 4 compared to control cells. The stimulated macrophages utilized endogenously released arachidonic acid for leukotriene B 4 and leukotriene C 4 synthesis. Exogenous arachidonic acid increased the formation of 12-hydroxyeico-satetraenoic acid (12-HETE) and 15-HETE and macrophages enriched with DHA or EPA produced similar amounts of 12-HETE and 15-HETE compared to control cells. These studies demonstrated that the synthesis of leukotriene C 4, leukotriene B 4 and HETE in macrophages is differentially affected by DHA and EPA.