The study of mucosal immunity has revealed that complex reciprocal interactions occur between intestinal intraepithelial lymphocytes (IEL) and intestinal epithelial cells (IEC). The present study focuses on the induction of inducible nitric oxide (NO) synthase in cocultures of freshly isolated rat IEL and the rat epithelial cell line IEC-18 after the addition of interleukin-1 (IL-1), tumor necrosis factor-, or lipopolysaccharide. When IEL and IEC were separated using Transwell chambers, NO synthesis was not induced, indicating that cell-cell contact was required. Culture of IEC-18 with IEL, even in the absence of inflammatory stimuli such as IL-1, resulted in upregulation of class I and II antigens on IEC-18, due to the interferon- (IFN-) that is constitutively produced by IEL. Addition of anti-IFN- antibody to the NO-producing cocultures resulted in inhibition of NO synthesis as well as the upregulation of class I and II antigen expression. These data indicate that IFN- production by IEL conditions IEC for the expression of other components of the inflammatory process.