Natural killer (NK) cells play an important role in the innate immune system by eliminating infected and mutated cells. Their cytotoxic capacities vary markedly among individuals. The cytotoxic activity can be measured in peripheral blood mononuclear cells (PBMCs) using the NK cell-specific target cell line K562. In this chapter, we present a protocol for the standardization and normalization of cell preparation and NK cell cytotoxicity measurement in a (51)Cr-release assay. By following these protocols, it is possible to compare the NK cell activity of numerous-if necessary selected-individuals in vitro.
