Cayman's Multidrug Resistance Assay Kit provides a convenient tool for studying the modulation of cellular multidrug resistance (MDR) machinery. The kit employs calcein AM, a cell-permeable nonfluorescent dye, which is cleaved intracellularly to yield the impermeant fluorescent molecule, calcein. This calcein is rapidly excluded from cells expressing the MDR proteins P-gp or MRP, and so functions as a probe for the detection of chemical compounds inhibiting MDR proteins. Propidium iodide is provided as a stain for marking dead or damaged cells. Cyclosporin A, a competitive inhibitor of MDR proteins, and verapamil, a noncompetitive inhibitor, are included as controls.