Adoptive cellular immunotherapy focuses on restoring cancer recognition via the immune system and improves effective tumor cell killing. Cytokine-induced killer (CIK) T cell therapy has been reported to exert significant cytotoxic effects against cancer cells and to reduce the adverse effects of surgery, radiation, and chemotherapy in cancer treatments. CIK can be derived from peripheral blood mononuclear cells (PBMCs), bone marrow, and umbilical cord blood. CIK cells are a heterogeneous subpopulation of T cells with CD3+CD56+ and natural killer (NK) phenotypic characteristics that include major histocompatibility complex (MHC)-unrestricted antitumor activity. This study describes a qualified, clinically applicable, flow cytometry-based method for the quantification of the cytolytic capability of PBMC-derived CIK cells against hematological and solid cancer cells. In the cytolytic assay, CIK cells are co-incubated at different ratios with prestained target tumor cells. After the incubation period, the number of target cells are determined by a nucleic acid-binding stain to detect dead cells. This method is applicable to both research and diagnostic applications. CIK cells possess potent cytotoxicity that could be explored as an alternative strategy for cancer treatment upon its preclinical evaluation by a cytometer setup and tracking (CS & T)-based flow cytometry system.