The discovery of drugs that can be used for the treatment of allergic disease is important in human health. Arctium lappa Linne (Compositae) (AL) has been used as a traditional medicine in Brazil and throughout Asia and is known to have an anti-inflammatory effect. In this study, the inhibitory effects of AL on degranulation and the release of mediators as well as on inhibition of cys-leukotriene biosynthesis by basophils were investigated. AL was selected out of 10,000 herbal extracts in a set-up for high throughput screening in which the degree of degranulation was monitored by the release of beta-hexosaminidase from rat basophil leukemia (RBL-2H3) cells. The AL extract significantly reduced degranulation and biosynthesis of cys-leukotrienes of human basophils in peripheral blood mono-nuclear cells (PBMCs) (50% inhibitory concentration [IC(50)] = 8.3 and 11.4 microg/ml, respectively). Viability and metabolic activity of the PBMCs were not affected. Although arctiin, the active component of AL that has been described in the literature, was not able to reduce degranulation in RBL-2H3 cells, a single high-performance liquid chromatography (HPLC) fraction from the AL extract inhibited beta-hexosaminidase release (IC(50) = 22.2 microg/ml). Topical administration of an aqueous extract of AL (5 mg/ear) on the ear of whey-sensitized mice 4 hrs before challenge with whey in the ear inhibited acute ear swelling by 50% in an in vivo cow's milk allergic model. The extract had no effect in this model when administered orally. In conclusion, the active component present in the active HPLC fraction of the AL extract was able to significantly reduce the release of inflammatory mediators through inhibition of degranulation and cys-leukotriene release in vitro. In addition, this active component was able to inhibit acute skin response in mice in vivo, indicating that AL is a very promising natural component for use in anti-allergic treatment.