To understand the anti-allergic mechanism of compound K, which is a metabolite of ginsenoside Rb1, a main constituent of the root of Panax ginseng C.A. Meyer (family Araliaceae), its inhibitory effect against IgE-antigen complex IAC)-induced passive cutaneous anaphylaxis (PCA) reaction in mice and mRNA and protein expressions of allergic cytokines in lAC-stimulated RBL-2H3 cells were investigated. Orally administered ginsenoside Rb1 more potently inhibited PCA reaction when administered at 5 h prior to the lAC treatment than when administered at I h before. However, compound K orally administered 1 h before lAC treatment showed a more potent anti-PCA reaction effect than when treated at 5 h before. Orally administered ginsenoside Rb1 more potently inhibited PCA reaction induced by lAC in mice than intraperitoneally treated one, apart from orally administered its metabolite, compound K, which was more potent than the orally administered one. The compound K, a metabolite of ginsenoside Rb1, inhibited mRNA and protein expressions of IL-4 and TNF-${alpha}$ and the activation of their transcription factor NF-$kappa$B and MAPK in lAC-stimulated RBL-2H3 cells. These findings suggest that orally administered ginsenoside Rb1 may be dependent on its metabolism by intestinal microflora in the intestine and the compound K may improve allergic diseases by the inhibition of IL-4 and TNF-${alpha}$ expresseion.