Rhodamine 123 chloride (CAS 62669-70-9) is a cationic fluorescent dye targeting the mitochondrial membrane potential ( m) It is designed to selectively accumulate within the mitochondrial matrix by crossing the plasma membrane thus enabling detection of mitochondrial function and membrane integrity Rhodamine 123 chloride exerts its biological activity primarily by responding to changes in mitochondrial membrane potential producing yellow-green fluorescence upon accumulation in live-cell mitochondria In cell-based studies its fluorescence intensity directly reflects m but no specific IC50 or EC50 value or experimental cell line is reported Based on these properties Rhodamine 123 chloride holds research potential in measuring mitochondrial function assessing apoptosis evaluating ATP production and monitoring P-glycoprotein-mediated efflux in fluorescence microscopy and flow cytometry applications