Heat-labile enterotoxin (LT) from enterotoxigenic Escherichia coli is known to possess strong immunoregulatory potential in terms of inhibition of the induction of oral tolerance and adjuvanticity in oral immunization. We found that oral administration of an immunogenic peptide of LT [LT-B(26-45); spanning the residues 26-45 of LT-B] induced systemic unresponsiveness in BALB/c mice resulting in diminished serum IgG responses. It was also shown that the spleen (SP) CD4+ T cells of tolerized mice failed to proliferate, whereas the Peyer's patches (PP) CD4+ T cells responded to the peptide. RT-PCR revealed that the SP CD4+ T cells did not generate IL-2 mRNA, while the PP CD4+ T cells expressed significant levels of IFN-gamma, IL-2, IL-4, and TGF-beta mRNA. Adoptive transfer of LT-B-specific intraepithelial lymphocytes to the tolerant mice abrogated the tolerance. In the reversed mice, LT-B(26-45)-stimulated SP CD4+ T cells expressed significant levels of IFN-gamma, IL-2, IL-4, and IL-6 mRNA. These results indicate that PP CD4+ T cells induce oral tolerance due to systemic T cell anergy.