The B subunit of E. coli heat-labile enterotoxin (EtxB) protects against the development of T helper type 1 (Th1)-mediated autoimmune pathologies in mice. Protection was transferable with splenic CD4(+) T cells and was less effective following CD25 depletion; implying a T regulatory cell (Treg)-mediated process. We hypothesized that if this were the case, then EtxB would also control a Th2-mediated disorder. We tested the effect of EtxB treatment on asthma development in ovalbumin (OVA)-sensitized mice. EtxB treatment diminished eosinophilia in bronchoalveolar lavage samples, reduced OVA-specific immunoglobulin E and interleukin 4 production locally and systemically, and reduced airway hyper-reactivity. EtxB induced a dose-dependent increase in Foxp3(+)CD4(+) T cells, and adoptive transfer of splenic CD4(+) T cells partially suppressed lung pathology. Importantly, EtxB treatment increased OVA-specific CD4(+)Foxp3(+) T cells in the lung and systemically. These data demonstrate that EtxB modulates the differentiation of allergen-specific T cells causing inducible Treg induction and preventing disease.