The calcein-AM dye used to stain the living cells is shown to have a low spontaneousleakage rate less than 15% in 4 hours at 37°C. Dilutions of targets stained by calcein-AM has a linear relationship with measured fluorescence values. NK cells, LAKs, and CTLs are readily detectable by this microtest. Quantitation of killing and kinetic analysis is readily performed with the test system. Calcein-AM is pH independent, better retained and more photostable. In addition, the high level of intracellular retention of calcein-AM and its low-level release after incorporation exclude possible cell-monolayer labeling and allow its use in a cell-cell interaction assay. Moreover, the bright fluorescence can easily be detected and measured by a microplate fluorescence reader. Calcein-AM is a highly lipophilic vital dye that rapidly enters viable cells, is converted by intracellular esterases to calcein that produces an intense green (530-nm) signal, and is retained by cells with intact plasma membrane. From dying or damaged cells with compromised membrane integrity or from cells expressing multidrug resistance protein (MRP), unhydrolyzed substrates and their fluorescent products are rapidly extruded from cells. The calcein-AM assay has been used to assess the cell viability, cytotoxicity and tp quantitate apoptosis.

Calcein-AM is found to be suitable for in vivo studies, because it has no deleterious effects on cell function and is, indeed, a marker of cell viability.