Abstract: The anti-anaphylactic activity of the osthol extracted from the fruits of Chidium monnieri (L.) Cusson was investigated. A single oral administration of osthol (100～200 mg/kg) inhibited the passive cutaneous anaphylaxis (PCA) reaction in mice and showed prominent protective effect against bronchial asthma induced by inhalation of histamine in conscious guinea pigs (P<0.01). In guinea pigs sensitized with egg albumin, significant inhibitory effect on spasmodic contraction of the guinea pigs ileum induced by antigen challenge in vitro was observed. At a concentration of 20 or 40 μg/ml, the substance blocked and counteracted the contractive activity of SRS-A on the isolated guinea pig ileum. Besides, osthol exhibited inhibitory effect on degranulation of mast cells, and the degree of inhibition was 78.6%.