Based on fluorometric determination of a dipeptide histidil-leucine, which is splitted off from a synthetic substrate (Cbz-Phe-His-Leu) by carboxycathepsin (carboxydipeptidyl peptide hydrolase), a method was developed for estimation of the enzymatic activity in human blood serum. The method is characterized by simplicity and high sensitivity; Use of small amount of blood serum (about 0.03 ml) was possible. In normal human blood serum the carboxycathepsin activity varied from 7.5 to 18 nmoles of the dipeptide, liberated per mg of protein per hr.