The purpose of this study was to determine whether the B subunit of cholera toxin (CtxB) has adjuvant activity over and above serving as a carrier protein for orally administered vaccines. An oligonucleotide that encodes an antigenic determinant (GtfB.1) from the glucosyltransferase B gene (gtfB) of Streptococcus mutans was genetically fused to the 5' terminus of either the CtxB gene (ctxB) or the Escherichia coli alkaline phosphatase gene (phoA). The resulting chimeric proteins were expressed in a phoA mutant strain of E. coli and then purified. The antigenicities of the proteins were confirmed by immunoblotting analysis using antisera specific for GtfB, CtxB, or PhoA. An equimolar amount of peptide on each carrier was administered by gastric intubation to mice three times at 10-day intervals. Antibody titers to the peptide, CtxB, and PhoA (in the serum, intestine, vagina, saliva, and bronchus) were determined by enzyme immunoassay. Antibody to the peptide was detected only in the sera of mice immunized with the peptide fused to CtxB. No antipeptide antibody was detected in mice immunized with the peptide fused to PhoA. The lack of detectable levels of antipeptide antibody in intestinal lavage fluid was attributed to dilution of the sample beyond the sensitivity of the assay. This was confirmed by cultivation of Peyer's patch and mesenteric lymph node tissue from mice orally immunized with the GtfB.1::CtxB chimera. Using this method, antipeptide antibody was detected in the culture fluid. We conclude that CtxB possesses unique properties that allow it to act as more than a simple carrier protein.