The possible effect of sulfasalazine, 5-aminosalicylic acid, and acetyl-5-aminosalicylic acid on endogenous arachidonic acid release and metabolism was studied in human polymorphonuclear leukocytes (PMNs). A new in vitro assay was used by which [1-14C]arachidonic acid is incorporated by purified peripheral PMNs until steady state was obtained (5 hr). After preincubation with the test drugs prior to activation with calcium ionophore A23187, the released eicosanoids were isolated by extraction and thin-layer chromatography (TLC) and quantitated by autoradiography and laser densitometry. Median drug concentrations needed for 50% inhibition of leukotriene B4 and 5-hydroxyeicosatetraenoic acid (5-HETE) release was 4-5 mM (range 1-9 mM) for both sulfasalazine and 5-aminosalicylic acid. The acetylated derivative of 5-aminosalicylic acid was ineffective. The present data suggest that inhibition of arachidonic acid lipoxygenation may be an essential action of sulfasalazine and its active metabolite, 5-aminosalicylic acid. Interference with lipoxygenase enzymes, rather than a steroid-like inhibition of arachidonic acid release from intracellular phospholipids, seems to be the mode of action.