Suc-AAPF-pNA is a chromogenic substrate that can be cleaved by cathepsin G (Km = 1.7 mM), subtilisins, chymotrypsin (Km = 60 μM), chymase (Km = 4 mM), and cyclophilin, but not neutrophil elastase. Release of p-nitroanilide is monitored at 405-410 nm. This substrate can be used for inhibitor screening and kinetic analysis.