Intestinal mast cells are thought to contribute to the mucosal inflammation in ulcerative colitis and Crohn's disease through release of inflammatory mediators. Since sulfasalazine and its metabolite 5-aminosalicylic acid are effective therapeutic agents in inflammatory bowel disease and have been shown to inhibit generation of inflammatory products in other cells, we examined the effect of these agents in vitro on human intestinal mast cell mediator release. Sulfasalazine (5 x 10(-4)-10(-3) M) was found to significantly enhance goat anti-human IgE-induced histamine release from intestinal mast cells, which is the same response as seen in human blood basophils, whereas its metabolite 5-aminosalicylic acid was an effective inhibitor of stimulated histamine release in both mast cells and basophils. 5-Aminosalicylic acid also inhibited production of prostaglandin D2 by the stimulated intestinal mast cells. Sulfasalazine alone, without immunologic stimulation, did not induce histamine release from mast cells or basophils, but the enhancement of ongoing mast cell activation by sulfasalazine may explain some cases of adverse reactions to the drug. The inhibition of mast cell histamine release and prostaglandin generation by 5-aminosalicylic acid demonstrates a potential therapeutic modality of this agent.