Der f II is a major mite allergen consisting of 129 amino acid residues. Der f II contains six cysteine residues, suggesting the existence of three disulfide bonds which would stabilize this small protein. As the first step in revealing the relationship between the structure and the allergenic property of Der f II, the formation of disulfide bonds was examined. Der f II purified from Dermatophagoides farinae was treated with lysyl-endopeptidase or proline-specific endopeptidase, and the peptide fragments thus generated were separated by reverse phase high performance liquid chromatography. Determination of the amino acid sequence of each peptide collected in this way proved the existence of three disulfide bonds between Cys8 and Cys119, Cys21 and Cys27, and Cys73 and Cys78.