Cell permeable resorufin derivative used as a fluorogenic indicator for cell viability. Resorufin was selected as the fluorophore because it fluoresces at a longer wavelength and is amenable to modification at the hydroxyl group which is essential for its fluorescence. Incubation with esterase at pH 8.0 results in a 80-90 nm shift of emission max. (Ex/Em: 500/~593nm in 0.1 M Tris pH 8.0 (after cleavage by esterase)). This cell viability indicator is 2 times more sensitive than calcein-AM.