The metabolism of arachidonic acid (AA) in human neutrophils was studied by incorporation of 1-14C-AA, removal of excess 1-14C-AA, and stimulation of radiolabelled cells with A23187. Radiolabelled lipids were quantitated by extraction, thin-layer chromatography, autoradiography, and laser densitometry. Following 5 h of incubation with 1-14C-AA, the maximum amount of radioactivity was located in triglycerides, 70%, and phospholipids, 30%. Activation of the cells with calcium ionophore A23187 led to release of free AA, 58%, whereas AA-metabolites revealed mainly lipoxygenase (arachidonate 5 lipoxygenase, E.C. 1. 13. 11. 34) activity, 5-HETE 13%, LTB4 5%, with only small amounts of cyclooxygenase (prostaglandin synthase, E.C. 1. 14. 99. 1) metabolites, HHT 2%. Intra-assay coefficient of variation for release of metabolites was approximately 15%. A potent immunosuppressive agent, cyclosporine A (CS-A) was shown to be without any effect in AA-release and metabolism. This method is applicable to studies of both basic cell function in human disease and to further immunopharmacological investigations.