Natural killer (NK) cells are a subset of lymphocytes with a distinct morphology and the ability to kill certain target cells via one or more cytolytic mechanisms. Measurements of in vitro NK activity from heparinized whole blood, or lymphocytes isolated from human peripheral blood or tissue, may be performed by a variety of techniques. Procedures using standard 51Cr-release assays as well as nonradioactive labeling materials are described in detail in this article, and advantages and disadvantages of these methods are discussed. Quantification of assay results, as percentage of specific lysis, lytic units, and cytolytic units, is explained with attention given to the limitations of each of these calculations. Quality control (QC) procedures that are needed to assure reproducible, reliable NK-cell assays for serial monitoring are described, and suggestions are provided for developing a QC program for these assays. Since NK cells can perform multiple functions, and the mechanism of NK-cell-mediated cytotoxicity is complex, the rationale for selecting a particular assay to measure NK activity is discussed.