Dissipation of the mitochondrial membrane potential (DeltaPsi(m)) has been accepted as a hallmark of some apoptotic processes. Depending on the model studied, it can occur during the early stages of cell death or later on, after loss of DNA integrity. Discordant data in the literature could be the consequence of using improper probes such as rhodamine 123 (R123) or DiOC(6)(3), which are not always appropriate for the study of DeltaPsi(m). The lipophilic cation JC-1, a specific probe for measuring DeltaPsi(m), is currently the gold standard. Thanks to recently developed instruments and additional probes for cell surface and intracellular markers, it can be used in polychromatic flow cytometric assays to simultaneously detect DeltaPsi(m) along with other biological parameters.