Objective: To determine the mechanisms by which a traditional herbal medicine, Senkinnaidakusan (SKNS), controls Th2 responses, we examined the production of IL-12 by murine macrophages treated with SKNS. Results: Treatment with SKNS significantly increased TLR4 mRNA in macrophages. Furthermore, pre-treatment with SKNS enhanced the production of IL-12 by macrophages stimulated with LPS. When SKNS was orally administered to C3H/HeN mice at the induction phase after OVA sensitization, the serum levels of OVA-specific immunoglobulin (Ig)E and IgG1 decreased, Interleukin (IL)-4 production by spleen T cells in response to OVA was significantly suppressed, while interferon (IFN)-gamma production was increased. After nasal challenge of OVA, eosinophilic infiltration in the nasal mucosa and the number of sneezes were significantly inhibited in SKNS-treated mice compared with control mice. Besides, expression of IL-5 in the nasal mucosa was also inhibited. Using another strain of mice, C3H/HeJ (TLR4 negative), there was no difference in OVA-specific Igs or splenic cytokine production between the SKNS treatment and non-treatment groups. The eosinophilic infiltration in the nasal mucosa, the number of sneezes and IL-5 expression in the nasal mucosa were also not effected even after SKNS treatment. Conclusion: These results suggest that oral administration of SKNS inhibits Th2 responses by enhancement of IL-12 release from macrophages via up-regulation of TLR4 expression.