It is widely used as a cell viability stain to differentiate live cells (which fluoresce green) from dead cells (which remain unstained) in biological research environmental microbiology and histology.

Its purpose is to assess cell viability and membrane integrity by measuring both enzymatic activity (needed to activate fluorescence) and the ability of intact cells to retain the fluorescent product.

Fluorescein diacetate (FDA) is a non-fluorescent lipid-soluble ester derivative of fluorescein that permeates cell membranes and is hydrolyzed by intracellular esterases to release highly fluorescent green fluorescein.