Interleukin (IL)-18, initially described as interferon (IFN)-γ-inducing factor, is expressed in the inflamed mucosa of patients with Crohn's disease. To investigate the role of IL-18 in intestinal inflammation, the effect of neutralizing antimurine IL-18 antiserum in dextran sulfate sodium (DSS)-induced colitis in BALB/c and C57BL/6 mice was examined. During a dose response of DSS, levels of colonic IL-18 increased parallel with clinical worsening. With the use of confocal laser microscopy, the increased IL-18 was localized to the intestinal epithelial layer. Anti-IL-18 treatment resulted in a dose-dependent reduction of the severity of colitis in both BALB/c and C57BL/6 mice. Colon shortening following DSS-induced colitis was partially prevented in the treatment groups. In the colon tissue homogenates, IFN-γ concentrations were lower in the anti-IL-18-treated DSS-fed mice compared with untreated DSS-fed mice. This suppressive effect of anti-IL-18 administered in vivo was also observed on spontaneous tumor necrosis factor-α, IL-18, and IFN-γ production from ex vivo colon organ cultures. The stimulation of lamina propria mononuclear cells by IL-18 and IL-12 resulted in a synergistic increase in IFN-γ synthesis. These findings suggest that IL-18 is a pivotal mediator in experimental colitis.