To obtain a new amylin peptide, having a specific amino acid sequence, intramolecular bonds between cysteine residues at specified sites and having amidated C-terminal tyrosine, separable from the pancreas of a patient suffering from diabetes and useful for diagnosing diabetes mellitus, etc.

This new amylin peptide has an amino acid sequence represented by the formula, intramolecular bonds between cysteines at the 2- and the 7-positions and an amidated C-terminal tyrosine, comprises one component in amyloid particles accumulated in Langerhans' islands of the pancreas of a patient suffering from diabetes and is usable as a peptide unique to diabetes mellitus (referred to as type II diabetes or non-insulin-dependent type diabetes mellitus) for diagnostic tests, etc., of the patient suffering from the diabetes containing an abnormal amount of the amylin in blood plasma with an antibody thereto. The peptide is obtained by homogenizing the pancreas of the patient suffering from the diabetes, separating the amyloid particles from the resultant homogenate, initially carrying out the get filtration according to a high performance liquid chromatography(HPLC), then treating the resultant filtrate by a reversed phase HPLC and a normal phase HPLC and conducting the fractionation.