fluorogenic substrates for proteolytic enzymes having the formula : ## str1 ## or acid salts thereof wherein : r 1 is hydrogen - l , hydrogen - d , benzoyl , benzenesulfonyl , glutaryl , pyroglutamyl , carbobenzoxy , d - serine , or carbobenzoxy - serine ; r 2 is hydrogen , phenyl , a straight , branched or cyclic alkyl having 1 to 4 carbons , or propionic acid ; r 3 is hydrogen , straight or branched or cyclic alkyl having 1 to 4 carbons , 4 - aminobutane , or 3 - guanidylpropane ; r 4 is methyl , 4 - aminobutane , or 3 - guanidylpropane ; r 5 is a fluorogenic moiety , severable from said compound by a proteolytic enzyme and having different fluorescent property when severed from said compound than when forming part of said compound . the enzymes , when reacting with the substrate , remove the fluorogenic group r 5 producing an increase in its fluorescence . the increase of the fluorescence is an indication of the activity of the enzyme present . specific enzymes thusly detectable include plasmin , thrombin , the factors x a , xi a , and xii a , kallikrein , trypsin , elastase , urokinase , and cathepsin b 1 .