A balance between production and degradation of reactive oxygen species (ROS) is critical for maintaining cellular homeostasis. Increased levels of ROS during oxidative stress are associated with disease conditions. Antioxidant enzymes， such as extracellular superoxide dismutase (EC-SOD)， in the extracellular matrix (ECM) neutralize the toxicity of superoxide. Recent studies have emphasized the importance of EC-SOD in protecting the brain， lungs， and other tissues from oxidative stress. Therefore， EC-SOD would be an excellent therapeutic drug for treatment of diseases caused by oxidative stress. We cloned both the full length (residues 1-240) and truncated (residues 19-240) forms of human EC-SOD (hEC-SOD) into the donor plasmid pFastBacHTb. After transposition， the bacmid was transfected into the Sf9-baculovirus expression system and the expressed hEC-SOD purified using FLAG-tag. Western blot analysis revealed that hEC-SOD is present both as a monomer (33 kDa) and a dimer (66 kDa)， as detected by the FLAG antibody. A water-soluble tetrazolium (WST-1) assay showed that both full length and truncated hEC-SOD proteins were enzymatically active. We showed that a potent superoxide dismutase inhibitor， diethyldithiocarbamate (DDC)， inhibits hEC-SOD activity.