MitoPTTM /TMRM ΔΨm depolarization detection kits are easy to use for screening cells by flow cytometry, fluorescence microscopy, and fluorescence plate reader assay formats. Inside a healthy, non-apoptotic cell, the lipophilic TMRE orTMRM dye, bearing a delocalized positive charge, enters the negatively charged mitochondria where it accumulates in an inner-membrane potentialdependent manner. The MitoPT potentiometric dyes exhibit very low toxicity and display rapid and reversible membrane equilibration properties. When the mitochondrial ΔΨm collapses in apoptotic cells, the MitoPT TMRE andTMRM potentiometric dyes no longer accumulate inside the mitochondria and become more evenly distributed throughout the cytosol. When dispersed in this manner, overall cellular fluorescence levels drop dramatically and this event can easily be visualized by fluorescence microscopy or quantitated by flow cytometry or fluorescence plate reader analysis techniques. The MitoPT ΔΨm depolarization detection kit easily distinguishes between healthy/non-apoptotic cell populations, and those cell populations that are transitioning into an apoptotic state.