A murine model of aerosol inhalation, leading to sensitization to birch pollen (BP) and its major allergen Bet v 1, was established in order to try to influence type-I allergic immune responses via the mucosal route. We previously demonstrated that simultaneous inhalation of BP and cholera toxin, a potent mucosal adjuvant, induced a Th1-like immune response to the allergen in naive mice and modulated allergic immune responses in sensitized mice. In contrast to cholera holotoxin, mucosal application of the cholera B subunit (CTB) conjugated to antigen has been shown to induce peripheral tolerance in certain models of Th1-based autoimmune diseases. In the present study we investigated the potential of such an antigen delivery system to suppress Th2-based, allergic immune responses. Mucosal administration of CTB/Bet v 1 conjugates prior to sensitization led to significantly increased allergen-specific IgE/IgG1 and IgG2a antibody levels and cytokine production (IL-5, IFN-gamma) in vitro. Thus, CTB coupled to Bet v 1 acted as an adjuvant rather than a tolerogen. On the other hand we noted that mucosal application of CTB coupled to ovalbumin led to marked suppression of antigen-specific IgE antibody levels and IL-5 production in vitro and thereby restricted allergic sensitization. These results indicated that the effects of CTB/antigen conjugates depended on the nature of the antigen. In contrast to Bet v 1 coupled to CTB, nasal as well as oral application of low doses of unconjugated, Bet v 1 prior to aerosol sensitization inhibited allergen-specific antibody responses of all isotypes, cutaneous type-I skin tests in vivo as well as allergen-specific lymphoproliferative responses and cytokine production (IL-4, IL-5, IL-10, IFN-gamma) in vitro, suggesting that both T- and B-cell tolerance to the allergen were induced. Taken together, mucosal tolerance induction as well as the use of certain transmucosal antigen delivery systems might be promising new strategies to modulate type-I allergic immune responses