MeOSuc-Ala-Ala-Pro-Val-AMC (MeOSuc-AAPV-AMC) is a fluorogenic substrate for human leukocyte and porcine pancreatic elastase. The methoxy succinyl (MeO-Suc) peptide AAPV is avidly hydrolyzed by these elastases but not by cathepsin G. The addition of 7-amino-4-methylcoumarin (AMC) generates a peptide of low fluorescence (excitation: 355-380 nm; emission 440-460 nm) that demonstrates a linear increase in fluorescence in the presence of elastase. This peptidyl-AMC substrate detects as little as 11 pM human leukocyte elastase and 18 pM porcine pancreatic elastase.