Immune (γ) Interferon is a substance produced by immunologically activated mononuclear cells. Besides its antiviral activity, interferon γ has a crucial role in immunoregulation, by acting directly upon lymphocytes and monocytes, and interacting with other soluble mediators of the immune response. Studies of the interferon system in inflammatory bowel disease have been limited, and little information is available on the generation of interferon during immunological events occurring in the human gut. To investigate the capacity of intestinal mucosal mononuclear cells to produce interferon γ, lamina proprial mononuclear cells, isolated from Crohn's disease, ulcerative colitis, and control patients, were incubated with interleukin 2 or phytohemagglutinin, and the amounts of interferon γ present in the culture supernatants were measured by a virus cytopathic effect inhibition assay. Under identical stimulatory conditions, culture supernatants of cells derived from actively involved mucosa of inflammatory bowel disease specimens contained two- to fivefold less interferon γ than those of cells from control tissue. However, the amount of interferon γ present in supernatants of cells from uninvolved inflammatory bowel disease mucosa was similar to that found in control supernatants. These results indicate that, in patients with active Crohn's disease and ulcerative colitis, mononuclear cells produce decreased amounts of interferon γ in the intestinal mucosa. The exact significance of these findings is unclear, but because of the importance of interferon γ in a variety of cell-mediated immune phenomena, its impaired availability might be relevant to the pathogenesis of inflammatory bowel disease.