To evaluate the antiallergic effect of the dried root of Astragalus membranaceus Bunge (AM) (Leguminosae), which inhibited the mouse passive cutaneous anaphylaxis (PCA) reaction in a preliminary experiment, its main constituents, astragalosides I and IV, were isolated and their antiallergic effects were investigated. Astragalosides I and IV inhibited the PCA reaction induced by the IgE-antigen complex, and the scratching behaviors induced by compound 48/80. These constituents reduced the protein expressions of $TNF-{alpha}$ and IL-4 in IgE-induced RBL-2H3 cells. These findings suggest that astragalosides I and IV as well as AM can improve IgE-induced anaphylaxis and scratching behaviors.