Hevein (Hev b 6.02) is a major IgE-binding allergen in natural rubber latex and manufactured products. Both tryptophans (Trp(21) and Trp(23)) of the hevein molecule were chemically modified with BNPS-skatole (2-nitrophenylsulfenyl-3-methyl-3(')-bromoindolenine); derivatized allergen failed to significantly inhibit binding of serum IgE in ELISA assays. Similarly, skin prick tests showed that hevein-positive patients gave no response with the modified allergen. Dot blot experiments carried out with anti-hevein mono- and polyclonal antibodies confirmed the importance of Trp(21) and Trp(23) for antibody-recognition, and demonstrated the specific cross-reactivity of other molecules containing hevein-like domains. We also report the structure of Hev b 6.02 at an extended resolution (1.5A) and compare its surface properties around Trp residues with those of similar regions in other allergens. Overall our results indicate that the central part of the protein, which comprises three aromatic and other acidic and polar residues, constitutes a conformational epitope.