A fluorimetrie assay of dipeptidyl carboxypeptidase is described. It involves incubation at 37 °C with the substrate, Z-Phe-His-Leu, and reaction of the dipeptide His-Leu which is released upon enzymatic hydrolysis, with o-phthalaldchyde to yield a fluorescent compound. The method is simple, precise and sensitive. The assay in serum is conveniently performed on 20-μl samples. Normal values in human serum range from 0.04 to 0.22 U/l.