Proteinase 3 (PR3, myeloblastin) is a polymorphonuclear leukocyte serine proteinase that degrades matrix proteins including fibronectin, laminin, vitronectin, and collagen type IV to generate antimicrobial peptides. Neutrophil elastase is a serine proteinase that is secreted by neutrophils during inflammation to destroy pathogens. Evaluating these enzymes is helpful to understanding inflammatory autoimmune processes. MeOSuc-AAPV-pNA is a highly sensitive peptide substrate that is hydrolyzed by both human and mouse neutrophil elastase and PR3, but not cathepsin G or chymotrypsin. Enzyme activity can be quantified by colorimetric detection of free p-nitroanilide at 405 nm.