A purified Ig-E binding immunogenic polypeptide the amino acid sequence of which consists of SEQ ID NO:1 is disclosed. The purified amino acid sequence is a major allergen of the walnut Juglans regia. Also described are a diagnostic test kit and use of the purified polypeptide in tests for identifying patients having an allergy to walnuts. Mutations of the disclosed polypeptide, as well as nucleic acid sequences encoding therefor are claimed as part of the invention.