The anti-inflammatory activity of Chungkukjang, a Korean traditional fermented soybean food (Korean-style Natto), was examined for the first time. From the results, it was found that the ethanol extract of Chungkukjang inhibited 5-lipoxygenase from A23187-treated RBL-1 cells, reducing leukoriene production (50% inhibitory concentration = 54.1 microg/mL). However, it did not greatly affect cyclooxygenase-2-catalyzed prostaglandin E(2) and inducible nitric oxide synthase-catalyzed nitric oxide production from lipopolysaccharide-treated RAW 264.7 cells at the same concentration ranges. Since leukotrienes are intimately involved in some allergic disorders, the anti-allergic activity of Chungkukjang was further examined in animal models of passive cutaneous anaphylaxis (type I hypersensitivity) and arachidonic acid-induced ear edema, two well-known in vivo models sensitive to 5-lipoxygenase inhibitors. After oral administration for 5 consecutive days, Chungkukjang significantly reduced (27.3%) passive cutaneous anaphylaxis in rats at 400 mg/kg/day. It also showed in vivo anti-inflammatory activity against arachidonic acid-induced mouse ear edema. Therefore, it is suggested that Chungkukjang may be beneficial for several allergic conditions such as asthma and atopic dermatitis.