The development of a new anticancer drugs targeted at energy metabolism of tumor cells is a promising­ approach for cancer treatment. The aim of our study was to investigate the action of thiazole derivative N-(5-benzyl-1,3-thiazol-2-yl)-3,5-dimethyl-1-benzofuran-2-carboxamide (BF1) and its complex with PEG based polymeric nanoparticle (PEG-PN) on respiration and mitochondrial membrane potential in murine NK/Ly tumor cells. The rate of oxygen uptake in NK/Ly cells was recorded by a polarographic method using a Clark electrode. The mitochondrial potential relative values were registered using fluorescence TMRM dye. No changes in glucose-fuelled basal respiration or maximal FCCP-stimulated respiration was detected after 15-min incubation of cells with BF1 (10 µM), PEG-PN or BF1 + PEG-PN complex Fluorescent microscopy data showed that BF1 or PEG-PN separately had no effect on the value of mitochondrial membrane potential, while BF1 + PEG-PN complex caused a significant decrease in mitochondrial membrane potential, indicating­ on the decrease of NK/Ly cells viability. Keywords: cell respiration, mitochondrial membrane potential, NK/Ly tumor cells, PEG, polymeric nanoparticles, thiazole derivative