Secretory antibodies in mucosal surfaces are known to play an essential role in protection against various infectious diseases. To enhance the production of such antibodies, influenza HA vaccine was inoculated intranasally into rabbits, together with cholera toxin B subunit (CTB) which is known to augment antibody response to an unrelated antigen. This combination resulted in high levels of serum IgG antibody responses against HA and CTB molecules, 3-4 weeks after inoculation, compared with the inoculation of HA vaccine alone. The adjuvant mechanism for CTB was studied by using Ussing chambers, in which nasal mucosa from rabbits were mounted. CTB was found to enhance the transepithelial flux of HA vaccine, from the mucosal side (lumen) into the serosal side (lamina propria), indicating that the permeability of the membrane was changed by CTB. Moreover, to achieve effective flux of HA vaccines, some interactions between the vaccine and CTB across the membrane were found, which may effect the effectiveness of the vaccine formulation. The results suggest that one of the mechanisms by which CTB enhances the production of mucosal antibody response is to enhance the transepithelial influx of vaccine into the nasal mucosa, where the cells involved in the antibody production are located. CTB may be used as a potent adjuvant to induce antibody response, by nasal vaccination, against pathogens impinging on mucosal surfaces.