Cobalt oxide (Co3O4) nanoparticles have been extensively used in various industrial and medical applications due to their special optical, magnetic, and electrical activity features. However, there is a lack of information about their toxicity and adverse effects on human health, especially concerning the kidney, which is considered to be a secondary target organ. We investigated the toxic potentials of Co3O4 nanoparticles on NRK-52E kidney epithelial cells by in vitro assays. CO3O4 nanoparticles were taken up by the kidney cells, and caused a decrease in cell viability, by significantly inducing apoptosis/necrosis at 100 mu g/mL. However, no significant DNA damage was observed. Co(3)O(4)nanoparticles induced cellular toxicity in kidney cells. These results should raise concern about the safety of Co3O4 nanoparticles in their various applications. Further studies are needed to elucidate their toxic mechanism.