Fluorescein di-β-D-glucopyranoside is one of the most sensitive substrates for glucosidases. The nonfluorescent substrate is sequentially hydrolyzed by β-glucosidase, first to fluorescein monoglucoside and then to highly fluorescent fluorescein. Enzyme-mediated hydrolysis of fluorescein di-β-D-glucopyranoside can be followed by the increase in either absorbance or fluorescence.