The isolation of two proteinases in crystalline form from extracts of beef pancreas has been described by Kunitz and Northrop (1). These enzymes have been named trypsin and chymotrypsin. Previous publications (2) from this laboratory have reported the finding of a series of synthetic peptide derivatives which were readily hydrolyzed by crystalline chymotrypsin. In this communication a synthetic substrate for crystalline trypsin is described. or-Benzoyl-Larginineamide hydrochloride (I) is hydrolyzed extremely rapidly by crystalline trypsin (recrystallized three times) to yield benzoyl-Z-arginine and ammonia (cf. Table I). This hydrolysis proceeds optimally at about pH 7.8 (Fig. 1).