The loss of mitochondrial membrane potential is a hallmark for apoptosis. It is an early event preceding phosphatidylserine externalization and coinciding with caspase activation. JC-1 Mitochondrial Membrane Potential Detection Kit is used to measure mitochondrial membrane potential changes in cells. In non-apoptotic cells, JC-1 accumulates as aggregates in the mitochondrial membranes, resulting in red fluorescence (590 nm). The brightness of red fluorescence is proportional to the potential and varies among different cell types. However, in apoptotic and necrotic cells, which have diminished mitochondrial membrane potential, JC-1 exists in the green fluorescent (529 nm) monomeric form. This kit provides a step-by-step protocol and ready-to-use reagents for performing assays for flow cytometry, fluorescence microscopy, or fluorometric microplate reader.