To obtain a new peptide for detecting homocystine useful for diagnosing arteriosclerosis, by subjecting the cystine residue of a peptide containing a cystine residue at a specific site of human serum albumin and homocystine to disulfide bond.

This new peptide is obtained by subjecting the cystine residue of a peptide containing an amino acid sequence composed of three or more continuous amino acid residues including a cystine residue at 34th of a human serum albumin of the formula and homocystine to disulfide bond and is used for simply detecting homocystine effective for diagnosing arteriosclerosis by binding assay of human serum albumin binding homocystine. The peptide is obtained by dissolving human serum albumin, L-homocystine and sodium boron hydride in 0.1M tris-hydrochloric acid buffer solution (pH6.5), reacting for one night while stirring and dialyzing.