Within the adaptive and innate immune system, effector lymphocytes known as cytotoxic T cells (CTLs) or natural killer (NK) cells play an essential role in host defense against tumor cells and virally infected cells. Here we describe a flow cytometry-based method to quantify CTLs or NK cell cytotoxic activity against melanoma cells. In this assay, spleen cells, peripheral blood mononuclear cells (PBMCs), or purified NK cell preparations are co-incubated at different ratios with a target tumor cell line. The target cells are pre-labeled with a fluorescent dye to allow their discrimination from the effector cells. After the incubation period, killed target cells are identified by a nucleic acid stain, which specifically permeates dead cells. This method is amenable to both diagnostic and research applications.