INTRODUCTIONIntact interchain and/or intrachain disulfide linkage in a protein can present problems during proteolytic or chemical fragmentation procedures. Disulfide bonds are commonly cleaved by reducing cystine to yield cysteine residues. However, cysteine residues are highly reactive, which can complicate sequence work. In this protocol, the intact protein (>1 mg) is reduced and then S-alkylated with iodoacetic acid (or iodoacetamide). The resulting cysteine derivative S-carboxymethylcysteine (or S-carboximadomethylcysteine) is easily detectable during chemical sequencing.