Linear B- and T-cell epitopes spanning all 103 amino acids of the Escherichia coli heat-labile toxin B subunit (LT-B) were assessed in mice orally immunized with native LT or with recombinant Salmonella enteritidis expressing LT-B. Oral administration of native LT induced mucosal immunoglobulin A (IgA) antibodies reactive with an epitope at residues 85 to 91, while IgA induced by recombinant Salmonella LT-B reacted with an epitope at residues 36 to 44. Serum IgG anti-LT-B antibodies from mice orally immunized with either LT or with recombinant Salmonella LT-B were directed to both epitopes. A single T-cell epitope spanning residues 34 to 42 was identified by T-cell proliferative and cytokine responses. When n 20-mer peptide (residues 26 to 45) with B- and T-cell epitopes was given orally to BALB/c (H-2(d)) and B10 congenic (I-A(d), I-Ab, and I-A(k)) mice, significant fecal IgA and serum IgG anti-LT-B antibodies were induced. The peptide also induced LT-B-specific T-cell proliferative responses in these mice. Orally administered LT-B peptide (residues 26 to 45) induced a cytokine profile indicative of both T helper 1- and 2-type cells. The remarkable immunogenicity of this 20-mer peptide makes it a candidate for a vaccine to protect against enterotoxigenic E. coli.