A major house dust mite allergen, Der p I, was isolated from spent growth medium and physicochemically characterized. These studies show that the allergen is monomeric, contains approximately 216 residues and 4 intra-chain disulphide bonds. The N-terminal amino acid is threonine. Circular dichroism studies show that the allergen contains 10% alpha-helical, 50% beta-pleated sheet and 40% random structures.