Sulfasalazine is an effective treatment in some inflammatory diseases that exhibit mast cell (MC) hyperplasia. However, its effect on MC has been incompletely studied. We have established that sulfasalazine inhibits the release of histamine and TNF-alpha from MC. Sulfasalazine and its metabolites, 5-aminosalicylic acid (5-ASA) and to a lesser extent sulfapyridine, inhibited Ag-stimulated histamine release from rat peritoneal MC in a concentration-dependent manner with a 50% inhibitory concentration of 6 x 10(6)M, 8 x 10(-6)M, and 3 x 10(-4)M, respectively. Similar results were observed with sulfapyridine and 5-ASA on Ag-stimulated histamine release of another population of MC, namely rat intestinal mucosal MC, but sulfasalazine was markedly less potent than its metabolites. Interestingly, sulfasalazine and sulfapyridine, but not 5-ASA, inhibited Ag-stimulated TNF-alpha released by MC. Similar results were observed with MC-mediated cytotoxic activity in which sulfasalazine and sulfapyridine, but nor 5-ASA, inhibited MC TNF-alpha-dependent cytotoxicity in a concentration-dependent manner. The addition of sulfasalazine to MC, up to 12 h after the cytotoxic assay (16 h) had started, significantly inhibited cytotoxic activity, suggesting that sulfasalazine inhibited the cytotoxic mediator, TNF-alpha. Indeed, affinity studies demonstrated that sulfasalazine binds TNF-alpha. Furthermore, the inhibition of MC cytotoxicity by sulfasalazine appeared to require new protein synthesis. Pretreatment of MC with sulfasalazine also inhibited the release of TNF-alpha and reduced the levels of TNF-alpha mRNA. Thus, sulfasalazine inhibits MC-mediated, TNF-alpha-dependent cytotoxicity by multiple mechanisms: competitive inhibition of soluble TNF-alpha, reduction of levels of TNF-alpha mRNA, and inhibition of TNF-alpha release.