Background/Aim: Recent studies indicate that chimeric antigen receptor (CAR)-T-cells seem to be superior to CAR modified NK-92 cells. One, at least partial, explanation to this discrepancy has been addressed herein, by having NK-92 cells as target cells in cytotoxicity reactions using peripheral blood mononuclear cells. Materials and Methods: A time-resolved fluorometric assay (TDA-labeled NK-92 or K562 as target cells) was used for measuring the cytotoxic activity of blood mononuclear cells (PBMC). Results: The cytotoxic capacity of the NK-92 cells was initially demonstrated by their ability to efficiently kill K562 cells. Interestingly, having PBMC as effector cells rendered the very same NK-92 cells sensitive to NK-cell mediated cytolysis. A 1:100 target:effector ratio gave 34.12.262 cells. Incubating PBMC for longer times (24 up to 48 h) potentiated their NK-activity against NK-92 cells even more, reaching a level close to that obtained with K562 cells. Conclusion: This study pinpoints a severe problem that has to be considered in future immune-based cancer therapies with NK-92 as well as CAR-transduced NK-92 cells.