Calcein Red? AM is a fluorogenic dye that is used to assess cell viability. As the acetoxymethyl ester, Calcein Red? AM is non-fluorescent but cell-permeant and is hydrolyzed to the fluorescent probe Calcein Red? by intracellular esterases. Calcein Red? is retained by living cells and displays excitation/emission maxima of 560/574 nm, respectively. Calcein Red? AM is a red-emitting variant of the green fluorophore calcein AM , which has been used to evaluate cell viability and cytotoxicity.