During the course of searching for anti-allergic substances from unexplored plant sources, an inhibitor of leukotriene $D_4$-induced tracheal contraction was isolated from Euphoribia helioscopia. This isolated polyphenol compound, known as helioscopinin-A, showed a certain inhibitory activity on capillary permeability in passive cutaneous anaphylaxis responses of rats and also on antigen-induced bronchial constriction in an experimental asthma model of guinea pigs. The compound at a high concentration weakly inhibited histamine release from isolated mast cells of rats. It is suggested that this compound is an anti-allergic and anti-asthmatic which exerts its activity through antagonism on leukotrene $D_4$-induced responses. A partial inhibition of allergic mediator relase may also bee involved.