A number of epidemiological and clinical studies have suggested an inverse association between allergy and helminth infection, such as Schistosomiasis. Therefore, we hypothesize that Schistosoma japonicum egg antigens, a type of native antigen, can induce production of CD4+ CD25+ T cells with regulatory activity, modulating airway inflammation and inhibiting asthma development. The frequency of CD4+ CD25+ T cells was determined by flow cytometry for mice treated with ovalbumin (OVA), CD25+ depletion/OVA, schistosome egg antigens, schistosome egg antigens/OVA and for control mice. The ability of CD25+ T cells from these mice to suppress T-cell proliferation and cytokine production was investigated both in vivo and in vitro. Results showed that the CD4+ CD25+ T cells of OVA-treated mice exhibited impaired control of dysregulated mucosal T helper 2 responses compared to the controls (P<0.05). Depletion of CD25+ cells accelerated OVA-induced airway inflammation and increased the expression of interleukin (IL)-5 and IL-4. Treatment with schistosome egg antigens increased the number and suppressive activity of CD4+ CD25+ T cells, which made IL-10, but little IL-4. In a murine model of asthma, S. japonicum egg antigens decreased the expression of Th2 cytokines, relieved antigen-induced airway inflammation, and inhibited asthma development. Thus, we provided evidence that S. japonicum egg antigens induced the production of CD4+ CD25+ T cells, resulting in constitutive immunosuppressive activity and inhibition of asthma development. These results reveal a novel form of protection against asthma and suggest a mechanistic explanation for the protective effect of helminth infection on the development of allergy.