The chymotrypsin inhibitor was isolated from honey locust (Gleditsia triacanthos L.) by chromatography on trypsin- and chymotrypsin-Sepharose 4B and by gel filtration on Sephadex G-75. The inhibitor can inhibit chymotrypsin but has no effect on trypsin, subtilisin and proteinases from Asp. oryzae and Str. griseus. The inhibitor forms a complex with chymotrypsin at a molar ratio of 1:1, has a molecular weight of about 20,000 and a low content of half-cysteine.