H2DCFDA [2′,7′-dichlorodihydrofluorescein diacetate] is a chemically reduced analogue of fluorescein used as an indicator for reactive oxygen species (ROS) in cells. Upon cleavage of the acetate groups by intracellular esterases and oxidation, the nonfluorescent H2DCFDA is converted to the highly fluorescent 2",7"-dichlorofluorescein.