Rhaponticin and chrysophanol 8-o-β-D-glucopyranoside isolated from the rhizomes of Rheum undulatum (Family Polygonaceae) are metabolized to rhapontigenin and chrysophanol, respectively, by human intestinal microflora. Most intestinal bacterial isolated from human feces catalyzed these metabolic pathways. Among rhaponticin and chrysophanol 8-o-β-D-glucopyranoside and their metabolites, rhapontigenin had the most potent inhibitory activity on a hyaluronidase, a histamine release from mast cell and passive coutaneous anaphylaxis (PCA) PCA reaction. The inhibitory activity of rhapontigenin was more potent than that of disodium cromoglycate, one of the commercial anti-allergic drugs. These results suggest that rhaponticin in the rhizomes of R. undulatum is a prodrug that has an extensitve anti-allergic property.