Disruption of mitochondrial transmembrane potential is one of the earliest intracellular events that occurs upon induction of apoptosis. The Mitochondrial Apoptosis Staining Kit provides a simple and sensitive, fluorescent-based in vitro assay for detecting the mitochondrial changes in apoptosis and distinguishing between healthy and apoptotic cells. The assay utilizes a cationic dye (called MitoStain) that fluoresces differently in healthy versus apoptotic cells. In healthy cells, MitoStain accumulates and aggregates in the mitochondria – fluorescing brightly red. In apoptotic cells, a shift in mitochondrial transmembrane potential prevents the dye from accumulating in the mitochondria. Instead it remains in the cytoplasm in its monomer form – fluorescing green. The fluorescent signals can be easily detected by fluorescence microscopy or flow cytometry using common filters and channels, respectively (FITC, rhodamine, PI). The assay takes approximately 20 minutes, is highly sensitive and detects apoptosis reliably in living cells.