CytoCalcein™ Violet 660 AM ester is designed for labeling live cells in the same way as Calcein, AM does. It is well excited with the Violet laser at 405 nm, which is now equipped in most flow cytometers. It is also well-excited by the excitation sources of fluorescence microscopes. Upon getting into live cells the CytoCalcein™ Violet 660 is hydrolyzed into a fluorescent dye of multiple negative charges, which makes the dye well retained in live cells. It has the largest Stokes Shift among the known live cell stains. This exceptional spectral separation from the typical existing FACS fluorophores provides additional options for multiplexing experiments with a flow cytometer or a fluorescence microscope. CytoCalcein™ Violet 660 has been developed for multiplexing flow cytometric and fluorescence imaging applications. It provides a new unique color for flow cytometric analysis of live cells.