The yeast saccharopine dehydrogenase (L-lysine-forming) contains an essential cysteine residue at the active site which can be carboxymethylated selectively by iodoacetate (Ogawa, H., Okamoto, M. and Fujioka, M. (1979) J. Biol. Chem. 254, 7030--7035). An undecapeptide containing this residue was isolated from the chymotryptic digest of the carboxymethylated enzyme by gel filtration chromatography and preparative paper electrophoresis. The amino acid sequence of the peptide was determined as Gly-Arg-Cys*-Gly-Ser-Gly-Ala-Leu-Ile-Asp-Leu, by the sequential Edman degradation and digestion with carboxypeptidases.