In this study, we measured the antiallergic activities of ginsenosides isolated from the root of Panax ginseng ( Araliaceae), and of their metabolites, as produced by human intestinal bacteria. Compound K, which was identified as a main metabolite, had the most potent inhibitory activity on beta-hexosaminidase release from RBL-2H3 cells and on the PCA reaction. The inhibitory activity of compound K was more potent than that of disodium cromoglycate, one of the commercial anti-allergic drugs. This compound demonstrated a membrane stabilizing action on differential scanning calorimetry. However, compound K did not inhibit the activation of hyaluronidase and did not scavenge active oxygen. These results suggest that the antiallergic action of compound K originates from its cell membrane stabilizing activity and that the ginsenosides of ginseng are prodrugs with extensive antiallergic properties. Abbreviations. compound K:20- O-beta- D-glucopyranosyl-20( S)-protopanaxadiol DNP:dinitrophenol DSCG:disodium cromoglycate DPPC:dipalmitoylphosphatidylcholine DPPH:1,1-diphenyl-2-picrylhydrazyl HSA:human serum albumin IC 50 :50% inhibitory concentration EC 50 :50% effective concentration XOD:xanthine oxidase ICR:Institute of Cancer Research PBS:phosphate buffered saline PCA:passive cutaneous anaphylaxis RAW264.7:mouse monocyte leukemiaRBL-2H3: rat basophil leukemia SD:Sprague-Dawley