When ginsenoside Rg5, a main component isolated from red ginseng, was incubated with three human fecal microflora for 24 h, all specimens showed hydrolyzing activity: all specimens produced ginsenoside Rh3 as a main metabolite, but a minor metabolite $3{eta},12{eta}$-dihydroxydammar-21(22),24-diene (DD) was observed in two specimens. To evaluate the antiallergic effect of ginsenoside Rg5 and its metabolites, the inhibitory effect of ginsenoside Rg5 and its metabolite ginsenoside Rh3 against RBL-2H3 cell degranulation, mouse passive cutaneous anaphylaxis (PCA) reaction induced by the IgE-antigen complex, and mouse ear skin dermatitis induced by 12-O-tetradecanoilphorbol-13-acetate (TPA) were measured. Ginsenosides Rg5 and Rh3 potently inhibited degranulation of RBL-2H3 cells. These ginsenosides also inhibited mRNA expression of proinflammatory cytokines IL-6 and $TNF-{alpha}$ in RBL-2H3 cells stimulated by IgE-antigen. Orally and intraperitoneally administered ginsenoside Rg3 and orally administered ginsenoside Rg5 to mice potently inhibited the PCA reaction induced by IgE-antigen complex. However, intraperitoneally administered ginsenoside Rg5 nearly did not inhibit the PCA reaction. These ginsenosides not only suppressed the swelling of mouse ears induced by TPA, but also inhibited mRNA expression of cyclooxygenase-2, $TNF-{alpha}$, and IL-4 and activation of transcription factor NF-kB. These inhibitions of ginsenoside Rh3 were more potent than those of ginsenoside Rg5. These findings suggest that ginsenoside Rg5 may be metabolized in vivo to ginsenoside Rh3 by human intestinal microflora, and ginsenoside Rh3 may improve antiallergic diseases, such as rhinitis and dermatitis.