INTRODUCTIONDisulfide bond cleavage is a prerequisite to many peptide-mapping strategies. It facilitates protein unfolding and thus optimizes proteolytic digestion. It also simplifies the interpretation of peptide maps by removing possible peptide fragment(s) in the mixture that are held together by disulfide bonds. This protocol describes the performic acid oxidation method for cleaving disulfide bonds. It is convenient for determining the combined cysteine and cystine content and the total methionine content of a protein.