Fluorescein di-β-D-galactopyranoside (FDG) is one of the most sensitive substrates for galactosidases. Nonfluorescent FDG is sequentially hydrolyzed by β-galactosidase, first to fluorescein monogalactoside (FMG) and then to highly fluorescent fluorescein. Enzyme-mediated hydorlysis of FDG can be followed by the increase in either absorbance or fluorescence.