The antiallergic activities of ginsenosides, which were isolated from acid-treated ginseng (Panax ginseng, Araliaceae), and their metabolites by human intestinal bacteria were measured. Ginsenoside Rh2, which is a main metabolite, had the most potent inhibitory activity on beta-hexosaminidase release from RBL-2H3 cells and in the passive cutaneous anaphylaxis reaction. The inhibitory activity of ginsenoside Rh2 was more potent than that of disodium cromoglycate, a commercial antiallergic drug. This compound showed membrane stabilizing action upon differential scanning calorimetry and inhibited nitric oxide (NO) and prostaglandin E2 (PGE2) in lipopolysaccharide-stimulated RAW cells. However, this ginsenoside Rh2 did not inhibit the activation of hyaluronidase and did not scavenge active oxygen. These results suggest that ginsenoside Rh2 can exhibit antiallergic activity originating from cell membrane-stabilizing activity and antiinflammatory activity by the inhibition of NO and PGE2 production.