Calcein-AM is a fluorogenic, cell-permeant fluorescent probe that indicates cellular health. When the acetoxymethyl ester is intact, this probe is nonfluorescent until acted upon by nonspecific esterases present within the healthy, live cells. Cleaving the AM ester allows the probe to excite and emit at 488nm/ 520nm respectively. The signal of Calcein-AM is proportional to cell vitality, as esterase activity decreases in cells with poor vitality. This cell-permeant probe is not retained with fixation and peameabilization.