Antiallergic action and the effect on normal immune function of an oriental medical prescription Gegangjochohwangsinbutang (GJHB) water extract were examined in several experimental models. GJHB water extract at 600-1,800 mg/kg/day administered orally for one week significantly recuded the passive cutaneous anaphylaxis in rats in a dose-dependent manner. Whereas the extravasation induced by intradermal leukotriene D4 in rats was significantly inhibited by GJHB water extract at 600-1,800 mg/kg/day, po, for one week, but that by histamine or serotonin was not influenced. In a rat asthma model, the delayed airway resistance in ovalbumin-induced asthmatic response was significantly attenuated by GJHB water extract of 1,800 mg/kg/day, po, administered for one week. It was also found that GJHB water extract at 1 mg/kg concentration in vitro relaxed the isolated uinea pig trachea contracted by leukotrien D4, but not that by histamine. The above findings indicate that GJHB water extract can ameliorate the adverse effect of type Ⅰ allergy such as asthma. This pharmacological action seems to derive from an inhibition of GJHB water extract on leukotriene D4-related physiological change.