Natural killer (NK) cells are an important effector cell of innate immunity. Their interaction with susceptible target cells triggers NK cell cytotoxicity and the release of cytokines. Immunofluorescence flow cytometry-based assays are now the preferred methods for measuring NK cell responses. For these assays, assessment is made on NK cells (CD3(-)CD56(+) CD16(+)) within the viable lymphocyte gate, defined by the parameters of size (FSC) and granularity (SSC). Accordingly, NK cells that have not dissociated from target cells at the time of measurement, or that have undergone target cell-induced apoptosis, are excluded from the viable lymphocyte gate and therefore from analysis. This unit describes a protocol for assessing NK cell function in response to target cells (both natural killing and antibody-dependent cell cytotoxicity) based on the loss of NK cells from the lymphocyte gate. This target-induced NK loss (TINKL) should provide a sensitive measure of NK cell responses in a clinical laboratory setting.