The MitoProbe™ Transition Pore Assay Kit provides a proven method (Petronilli V. et al. 1989, 1999) of measuring mitochondrial permeability transition pore opening. The kit includes a dye and a quencher that are easily loaded into cells. In healthy cells, the dye, but not the quencher, enters mitochondria. The mitochondria remain brightly fluorescent until mitochondrial pore activation permits quenching of the fluorescence.