The carboxypeptidase inhibitor from Russet Burbank potatoes (C. A. Ryan et al. (1974b), J. Biol. Chem 249, 5495) is a mixture of approximately equal amounts of two polypeptide chains containing 38 and 39 amino acid residues, respectively. The chains differ in their amino terminal sequence only, one beginning with smaller than Glu-His-Ala ... and the other with smaller than Glu-Gln-His-Ala ..... Specific cleavage procedures utilized in determining the complete amino acid sequence of the inhibitor included acid cleavage of the aspartyl-proline bond and tryptic and chymotryptic digestion. Mass spectrometry, automatic Edman degradation, and subtractive Edman degradation were employed in sequencing the resulting peptide fragments.