The activity of (2'-5')oligoadenylate synthetase, an enzyme induced by and mediating the antiviral action of interferon, was measured in extracts of intestinal mononuclear and epithelial cells isolated from patients with Crohn's disease, ulcerative colitis, and a control group. No significant differences were detected among (2'-5')oligoadenylate synthetase activities of lamina propria mononuclear cells derived from inflammatory bowel disease-involved and histologically normal control mucosa. Similarly, epithelial cells from inflammatory bowel disease and control patients expressed comparable levels of the enzyme, but these were significantly higher (p less than 0.01) than those found in autologous mononuclear cells. These results indicate that interferon is locally produced along the human intestinal mucosa under normal and inflammatory conditions. While this study supports the contention that induction of an antiviral state does not play a significant role in the pathogenesis of inflammatory bowel disease, it does not exclude the activation of the interferon system for other immunologic functions.