5(6)-Carboxy-2’,7’-dichlorofluorescein diacetate is an oxidant-sensitive and cell-permeable fluorescent probe. Base hydrolysis of the ester bonds by intracellular esterases releases the acetate groups and the resulting compound, 5(6)-carboxy-2’,7’,-dichlorofluorescein, is oxidized by reactive oxygen species (ROS) and nitric oxide (NO) and displays excitation/emission maxima of 504 and 530 nm, respectively, which can be quantified as a measure of oxidant production.