N -succinyl- L -phenylalanine- p -nitroaniline has been used as a substrate for chymotrypsin hydrolyzing activity and in molar absorbance assays. It has also been used to monitor α-chymotrypsin inhibition assays in vitro. N -succinyl- L -phenylalanine- p -nitroaniline has been used as a substrate for chymotrypsin hydrolyzing activity and in molar absorbance assays. It has also been used to monitor α-chymotrypsin inhibition assays in vitro.