Cholera toxin (CT) and the analogous heat-labile enterotoxin (LT) from Escherichia coli have several immunomodulating effects which alone or in combination might explain their strong adjuvant action in stimulating mucosal IgA and other immune responses to admixed unrelated antigens after oral immunization. These effects include, depending on animal species and experimental systems, enhanced antigen presentation by a variety of cell types; promotion of isotype differentiation in B cells leading to increased IgA formation; and complex stimulatory as well as inhibitory effects on T-cell proliferation and lymphokine production. This adjuvant activity appears to be closely linked to the ADP-ribosylating action of CT and LT associated with enhanced cyclic AMP formation in the affected cells, and thus it may prove difficult to eliminate the enterotoxic activity without loss of adjuvanticity. However, through a separate mechanism, as an antigen-carrier system providing specific binding to epithelium including the M cells of intestinal Peyer's patches, both CT and its non-toxic binding subunit moiety (CTB) have been shown to markedly enhance the mucosal immune response to various foreign antigens or epitopes covalently linked to these molecules. This gives promise for the future use of CTB or related non-toxic binding derivatives as vehicles to facilitate induction of mucosal immune responses to a broad range of antigens for human vaccination purposes.