Thiazolyl Blue (MTT) is used in conjunction with rhodamine B to measure mitochondrial membrane potential. Thiazolyl Blue-formazan is generated in mitochondria acts as a fluorescence quencher for rhodamine that distributes across membranes of viable cells according to membrane potential. In the absence of mPMS, cellular reduction of Thiazolyl Blue is strong. Thiazolyl Blue is membrane-impermeable when incorporated into large unilamellar liposomes, and therefore it is taken up by cells via endocytosis.