A leucylalanyarginine derivative represented by the formula, ##STR1## wherein R.sub.1 represents hydrogen or an amino-protecting group; R.sub.2 and R.sub.3 represent hydrogen or guanidino-protecting groups; and R.sub.4 represents naphthyl. The above compound is useful as an excellent substrate for various enzymes, such as trypsin, plasmin, kallikrein, urokinase, Cl-esterase and the like. Accordingly, the activity of enzymes can be measured by use of said compound as a substrate.