To further define the role of mast cells in the idiopathic inflammatory bowel diseases, mediator release from intestinal mast cells derived from actively inflamed and relatively quiescent areas of ulcerative colitis was studied. It was hypothesized that mast cells in the actively diseased segments would indicate involvement in the disease process by releasing a different profile of mediators than cells in uninflammed tissue. Mast cell-containing suspensions derived from matched segments of 12 ulcerative colitis specimens were compared for responsiveness to the mast cell stimulus goat anti-human immunoglobulin E. Supernatants from challenged cells were analyzed for levels of three mast cell mediators, histamine, prostaglandin D2, and the sulfidopeptide leukotriene C. Mast cells from the actively involved areas released significantly greater amounts of histamine, prostaglandin D2, and sulfidopeptide leukotriene. The difference in histamine release was not a result of greater stores of histamine in the active tissue cells, because the total histamine content of the mast cells from the active areas was not significantly greater. The enhanced release of both preformed and newly generated mediators indicates activation of those cells in the course of the disease and points to the mast cell contribution to the inflammatory process in these disorders.