Recent concerns have emerged surrounding the toxicity that cobalt may represent when used in MOM implants. Owing to corrosion and wear of MOM implants, the subsequent released cobalt nanoparticles (CoNPs) or Co ions (Co2+) can cause adverse reactions, such as the generation of pseudotumors, extensive necrosis, early osteolysis, and implants failure. The present study confirmed that CoNPs and Co2+ can induce dose- and time-dependent cytotoxicity with increasing reactive oxygen species (ROS) levels. Additionally, using metallothionein (MT), a heavy metal-binding protein, the present study assessed the protective effects of Zn2+ against CoNPs and Co2+-induced cytotoxicity of RAW 264.7 cells through ROS pathway. Further studies are needed to explore the underlying protective mechanisms in vitro. However, the current findings indicate that the ROS pathway may be a potential target for therapeutic interventions. (C) 2017 Published by Elsevier Inc.