L-Leucine-β-naphthylamide is a chromogenic substrate widely used in biochemical and clinical research. It is a synthetic peptide derivative formed by linking L-leucine to β-naphthylamine via an amide bond. This compound is a specific and sensitive substrate for the enzyme leucine aminopeptidase (LAP). Its primary application is as a diagnostic reagent for the colorimetric assay of leucine aminopeptidase activity in biological samples, such as serum or tissue homogenates. Upon enzymatic hydrolysis by LAP, β-naphthylamine is released, which can be diazotized and coupled to a suitable agent (e.g., N-(1-naphthyl)ethylenediamine) to produce a measurable colored azo dye. This assay is historically significant for investigating liver function and certain pathological conditions. While its use in modern clinical diagnostics has been largely superseded by more advanced techniques, it remains a valuable tool in fundamental enzymology studies and for the validation of LAP activity in research settings. It also serves as a reference standard or process-related impurity in the context of diagnostic kit manufacturing or peptide synthesis.