Sulfasalazine is a potent antiinflammatory drug used in the treatment of ulcerative colitis. The mechanism of action of sulfasalazine is unknown but a recent study [W. F. Stenson and E. Lobos, J. clin. Invest. 69, 494 (1982)] demonstrated that sulfasalazine, at therapeutic concentrations, blocks human neutrophil lipoxygenase, suggesting that its antiinflammatory effects may be mediated in part by the inhibition of the synthesis of the chemotactic lipids 5-hydroxy-6,8,11, 14-eicosatetraenoic acid (5-HETE) and leukotriene B4 (LTB4). In the present study the effect of sulfasalazine on metabolism of exogenous arachidonic acid by human platelets was investigated. Sulfasalazine inhibited platelet thromboxane synthetase (IC50 = 0.9 mM) and partially inhibited cyclooxygenase. A methylated analog of sulfasalazine also inhibited thromboxane synthetase (IC50 = 0.3 mM) and partially inhibited cyclo-oxygenase. Neither of the cleavage products of sulfasalazine (5-aminosalicylate and sulfapyridine) inhibited thromboxane synthetase although 5-aminosalicylate blocked cyclooxygenase (IC50 = 5 mM). Neither sulfasalazine nor the methylated analog nor the cleavage products inhibited platelet lipoxygenase. This is in contrast to the inhibitory effects of sulfasalazine on neutrophil 5-lipoxygenase. The concentration of sulfasalazine in the colons of treated patients is several-fold greater than the IC50 for thromboxane synthetase.