To initiate studies on the significance of filarial arachidonic acid metabolism in the immunopathogenesis of human filariasis, we evaluated the ability of microfilariae of the human filarial parasite Brugia malayi to take up and incorporate exogenous arachidonate. When 4 X 10(5) microfilariae were incubated in vitro with 3 nM [3H]arachidonic acid for 0.2, 24, and 48 hr, 23%, 70%, and 75% of tritium activity were associated with microfilariae, respectively. [3H]arachidonic acid was taken up by viable but not by killed microfilariae. Electron microscopic autoradiographic examination of living microfilariae incubated with [3H]arachidonic acid demonstrated numerous tritium-induced silver grains over sectioned parasites. Chromatographic resolution and quantitation of classes of neutral lipids and phospholipids of parasites established that incorporated [3H]arachidonic acid was rapidly and almost completely esterified into these lipids, predominantly into phosphatidylinositol and phosphatidylcholine. Microfilariae, the blood-borne stage of B. malayi, possess the requisite biochemical pathways to rapidly take up and incorporate exogenous arachidonate.