BACKGROUND Extracellular superoxide dismutase (EC-SOD) is a principal enzymatic scavenger of the superoxide anion in extracellular spaces. It is thought to be important as a defense against superoxide-mediated damage to both cell surfaces and extracellular matrix proteins. EC-SOD may also be important in regulating intercellular signalling by extracellular superoxide. EC-SOD is believed to be mainly located in the extracellular matrix of tissues. However, the specific localization of EC-SOD is unknown. Knowledge of the distribution of EC-SOD is an essential step in defining its functions. METHODS Using light microscopic immunohistochemistry, electron microscopic immunocytochemistry and an EC-SOD affinity-purified polyclonal rabbit antibody to human recombinant EC-SOD, we evaluated the distribution of EC-SOD in human lungs. RESULTS These studies revealed that in the lung EC-SOD is primarily located in the extracellular matrix. Specifically, EC-SOD is found in areas containing high amounts of type I collagen and other unidentified matrix elements, but was not seen in areas rich in elastin or cartilage. In the lung, EC-SOD is predominantly located around larger vessels and airways, and, to a lesser degree, in the extracellular matrix around alveolar and capillary regions. Some EC-SOD was found in bronchiolar epithelial cell junctions and around the surface of vascular and airway smooth muscle cells. No labeling was seen on endothelial cell surfaces of capillaries, small muscular, or large elastic vessels in the lung. Labeling for EC-SOD was limited to the extracellular spaces, consistent with it being a secreted protein, with the exception of a small amount of intracellular labeling seen in bronchial epithelial cells and type II cells. CONCLUSIONS The labeling distribution of EC-SOD in human lungs was defined using immunohistochemistry and immunocytochemical techniques. The findings suggest that EC-SOD is not diffusely located throughout the lung, but has a specific distribution in the extracellular matrix. These findings on the distribution of EC-SOD suggest that specific roles for EC-SOD may include the protection of matrix elements such as collagen from oxidative stress and that it may also function in the regulation of intercellular signals that are modulated by reactive oxygen species.