The novel cytokine interferon-gamma-inducing factor ("interleukin-18") is produced by macrophage-like cells in mice with endotoxin shock and induces the production of interferon-gamma by T cells in vitro. To determine the physiological role for mouse interferon-gamma-inducing factor, we studied its tissue distribution in several organs (intestine, spleen, thymus, kidney, and liver) in healthy mice of different ages, including fetal stages. Activity of the cytokine in the organ extracts of adult mice was measured by enzyme-linked immunosorbent assay, and the cellular distribution of interferon-gamma-inducing factor in organs from fetal and adult mice was determined by immunohistochemistry. Intestinal extracts of adult mice showed the highest concentrations among the organs studied. Other organ extracts of adult mice showed lower concentrations of the cytokine. Immunohistochemical analysis revealed that interferon-gamma-inducing factor was localized in the cytoplasm of intestinal epithelial cells from fetal and adult mice. These results show for the first time that intestinal epithelial cells may be the main producers of interferon-gamma-inducing factor under normal physiological conditions and suggest that its constitutive expression in intestinal epithelial cells may have an important role in the induction of mucosal immunity.