Although organelles remain grossly normal during the earlier stages of apoptosis, research has revealed a pivotal role for mitochondria in the apoptotic cascade. The energy generated during cellular respiration accumulates in the mitochondrial transmembrane space as an electron gradient called the mitochondrial membrane potential or Δψm. This electrochemical gradient is often disturbed during apoptosis and can be detected using cationic dyes such as DePsipher™ (5,5’6,6’- tetrachloro-1,1’,3,3’-tetraethylbenz-imidazolylcarbocyanine iodide). DePsipher™ readily enters cells and accumulates as a multimeric aggregate within healthy mitochondria, and under UV light, this multimeric form emits red light. In apoptotic cells, the mitochondrial membrane potential collapses and the dye returns to a monomeric form. The monomeric molecule emits green fluorescence under UV light. DePsipher™ provides a rapid fluorescence-based assay for the apoptosis-associated loss of mitochondrial potential.