Succinyl-trialanine-p-nitroanilide (Suc-Ala3-pNA) was previously described as a convenient substrate for pancreatic elastase which is not split by trypsin and chymotrypsin. An enzyme activity splitting Suc-Ala3-pNA was found in the supernatant of the mucous membrane of human small intestine homogenized with Triton X-100.