Fluoresceinated casein is widely used for fluorometric measurement of protease activity. In the intact substrate, casein is heavily labeled with a fluorescein derivative, resulting in almost total quenching of the conjugate''s fluorescence. Protease-catalyzed hydrolysis relieves this quenching conjugate, yielding brightly green fluorescent dye-labeled peptides. The increase in fluorescence intensity is directly proportional to protease activity. The lightly labeled fluorescein casein may be useful for a continuous assay if monitored by fluorescence polarization.