OBJECTIVE Intestinal epithelial cells present protein antigens to primed T cells in vitro. The aim of this study was to investigate whether intestinal epithelial cells present peptide antigens in vitro and in vivo after oral administration. METHODS Small intestinal epithelial cells from naive LEW (RT1) rats pulsed in vitro with a synthetic immunogenic major histocompatibility complex allopeptide, RT1.Du beta 20-44, or in vivo by oral administration of the peptide were tested for their ability to induce specific proliferation of LEW T cells primed in vivo to RT1.Du beta 20-44. RESULTS In vitro pulsed intestinal epithelial cells induced specific proliferation of RT1.Du beta 20-44-primed T cells. Intestinal epithelial cells isolated from LEW ras that received a single oral dose of RT1.Du beta 20-44 18 hours earlier also induced specific proliferation of RT1.Du beta 20-44-primed LEW T cells. Furthermore, epithelial cells harvested from LEW rats that received WF (RT1u) splenocytes orally 18 hours earlier induced specific proliferation of RT1.Du beta 20-44-primed LEW T cells. CONCLUSIONS Intestinal epithelial cells take up processed alloantigen in vitro and in vivo for presentation as peptides to primed T cells. These observations provide a novel approach to study the role of the intestinal immune system in immune regulation in vivo.