Degradation impurities have been investigated, by means of high-performance liquid chromatography (HPLC)/ mass spectrometry, in commercial ampoules of salmon calcitonin, aqueous solution, stored for up to 2 years. Oxidation derivatives of the cysteine residues in positions 1 and 7, with cleavage of the disulfide bridge, were observed in all samples. Using our reversed-phase separation, these oxidation derivatives coeluted and presented an HPLC behavior identical to the derivative of salmon calcitonin with the disulfide bridge reduced. In a previous study this last compound was tentatively identified, without confirmation by mass spectrometry, as a major degradation product of salmon calcitonin in aqueous solution.