The Vybrant® Multidrug Resistance Assay Kit provides a rapid and simple method for large-scale screening of MDR inhibitors. This assay uses the fluorogenic dye calcein, AM as a substrate for efflux activity of Pgp. Calcein AM is nonfluorescent, highly lipid-soluble dye that can rapidly penetrate the plasma membrane of normal cells. Once inside the cell, ester bonds are cleaved by endogenous esterase, transforming calcein AM into the hydrophilic and intensely fluorescent dye, calcein. Multidrug resistant (MDR) cells expressing high levels of P-glycoprotein (Pgp) rapidly extrude nonfluorescent calcein AM from the plasma membrane, reducing the accumulation of fluorescent calcein relative to normal cells.