Aminopeptidases are proteolytic enzymes that remove L -amino acids sequentially from the amino termini of polypeptide chains. A number of aminopeptidases have been isolated, including leucine aminopeptidase from serine kidney cytosol (1 ), aminopeptidase P from E. coli (2 ), proline iminopeptidase from E. coli , and swine kidney (3 ), aminopeptidase B from rat liver (4 ), and aminopeptidase A from rat kidney (5 ). However, three aminopeptidases in particular have found routine use in protein chemistry. The first is pyroglutamate aminopeptidase, a thiol exoprotease that cleaves N-terminal pyroglutamyl residues (pyrrolidone carboxylic acid) from peptides and proteins (6 –10 ). N-terminal glutamine residues can readily cyclize to the pyroglutamyl derivative (Fig. 1 ). This can occur during peptide and protein purification (it is uncertain whether the N-terminal pyroglutamyl residues of a number of naturally occurring peptides and proteins are genuine posttranslational modifications, or were introduced by cyclization of N-terminal glutamine during purification) or during sequence determination when glutamine was the newly liberated N-terminal amino acid. This cyclized derivative does not have a free amino group, and therefore, the peptide or protein is not amenable to sequence determination, unless the pyroglutamyl derivative is removed by pyroglutamate aminopeptrdase (11 ,12 ).