Granzyme B is a serine protease of the chymotrypsin fold that mediates cell death by cytotoxic lymphocytes. It is a processing enzyme, requiring extended peptide substrates containing an Asp residue. The determinants that allow for this substrate specificity are revealed in the three-dimensional structure of granzyme B in complex with a macromolecular inhibitor. The primary specificity for Asp occurs through a side-on interaction with Arg 226, a buried Arg side chain of granzyme B. An additional nine amino acids make contact with the substrate and define the granzyme B extended substrate specificity profile. The substrate determinants found in this structure are shared by other members of this protein class and help to reveal the properties that define substrate specificity.