Background: IgE-binding peanut proteins smaller than 15 kDa were previously identified as potential allergens in the majority of our peanut allergic population. Objective: To characterize the novel allergen in order to determine whether it was similar to one of the thus far identified recombinant peanut allergens (Ara h 1-7). Methods: An IgE-binding protein of <15 kDa was purified and identified via N-terminal sequencing. Its IgE-binding properties were investigated using immunoblotting, basophil degranulation, and skin prick testing. Possible cross-reacting epitopes with other peanut allergens were studied using IgE-immunoblotting inhibition. Results: The purified protein is a monomeric protein with a molecular weight of 14981 Da as determined using matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopy. The amino acid sequence of the first 39 N-terminal residues is identical to that of Ara h 6, indicating that the allergen is Ara h 6. It is recognized by 20 out of 29 peanut-allergic patients on IgE-immunoblot, and its potent biological functionality is demonstrated by the degranulation of basophils, even at concentrations below 10 pg/mL, and by positive skin prick reactions. Ara h 6 has homology to Ara h 2, especially in the middle part and at the C-terminal part of the protein. Almost complete inhibition of IgE-Ara h 6 interaction with Ara h 2 demonstrates that at least part of the epitopes of Ara h 6 are cross-reactive with epitopes on Ara h 2. Conclusions: Peanut-derived Ara h 6 is a biologically active allergen recognized by the majority of our peanut-allergic patient population and can be considered a clinically relevant peanut allergen. © 2005 Blackwell Publishing Ltd. Chemicals / CAS: immunoglobulin E, 37341-29-0; Albumins; Allergens; Ara h 2 allergen, Arachis hypogaea; Ara h 6 allergen, Arachis hypogaea; Glycoproteins; Immunoglobulin E, 37341-29-0; Plant Proteins; Recombinant Proteins