Several immunoadjuvant systems have been proposed to enhance mucosal immune responses of orally administered purified antigens. Cholera toxin (CT) or its B subunit (CTB) have been found to promote immune responses to antigens when they are co-administered via mucosal routes. Oral administration of antigens incorporated into liposomes has also been shown to result in enhanced mucosal immune responses. Here, we describe the covalent coupling of CT and CTB to small unilamellar liposomes for targeting these vesicles to Peyer's patch M cells, following their oral administration. Conjugation was done by means of a thioether bond using succinimidyl(4-N-maleimidomethyl)cyclohexane-1-carboxylate to modify the dipalmitoylphosphatidyl-ethanolamine constituent of liposomes and N-succinimidyl-3-(2-pyridyldithio)propionate to thiolate CT or CTB. The biological activity of CT or CTB bound to liposomes was confirmed by a hemagglutination assay using GM1-enriched human erythrocytes. Furthermore, oral administration of CT-conjugated liposomes to rats resulted in the induction of serum IgG and salivary IgA anti-CT responses. CT-conjugated liposomes may prove to be a useful system for targeted delivery and immunoenhancement of weakly immunogenic antigens. © 1995.