The herbal medicine shoseiryu-to is an effective agent in the treatment of allergic rhinitis. However, the mechanism by which it exerts its action in improving patient symptoms remains unclear. It might affect the allergen-induced TH1 and/or TH2 responses. This study investigated whether the herbal medicine could affect cytokine synthesis by peripheral blood mononuclear cells (PBMCs) in response to the major Dermatophagoides farinae (D. farinae) allergen, Der f 1. PBMCs were obtained from 15 patients with perennial allergic rhinitis due to D. farinae, and were stimulated for 96 h with 10 micrograms/ml Der f 1 in the presence or absence of 45 mg/ml shoseiryu-to. The culture supernatants were harvested to determine the synthesis of IgE, interleukin 5 (IL-5), IL-6, IL-10, interferon-gamma (IFN-gamma) and tumour necrosis factor alpha (TNF-alpha). The agent did not affect the allergen-induced synthesis of IL-5, IL-6 and IFN-gamma, but somewhat decreased the synthesis of IgE and IL-10. This study highlighted an interesting pharmacological action of shoseiryu-to to substantially inhibit the allergen-induced synthesis of TNF-alpha. Our study suggests that the shoseiryu-to may alleviate nasal symptoms in allergic rhinitis through control of the allergen-induced inflammatory process.