Copper,zinc superoxide dismutase (CuZnSOD), an antioxidant enzyme, is unique in requiring two essential metals for catalytic function. Yet, only one, copper, seems to regulate the expression of functional activity. Restricting dietary copper quickly impairs catalytic functioning of CuZnSOD in numerous tissues. Diets supplemented with copper or small amounts of CuCl2 administered intraperitoneally restore the enzyme activity in animals deprived of copper. Thus, CuZnSOD has been considered a good marker of copper status. A metal-free (apo) form of CuZnSOD could exist in tissues at all times, but especially when an animal is deprived of copper. Restoring CuZnSOD activity with copper permits elucidation of the pathway of copper incorporation into the enzyme. Ceruloplasmin and albumin transport copper to the enzyme in vitro. K562 cells, a human erythroleukemic cell line, can extract copper from ceruloplasmin and incorporate it into CuZnSOD. Ascorbic acid stimulates the transfer of 67Cu transfer from ceruloplasmin to the cells, and somewhat unexpectedly, appears to restrict the amount of transferred copper that becomes bound to the enzyme. Reactivation of CuZnSOD in healthy individuals has the potential of being a useful tool for assessing copper status. This approach has merit, but one must consider that the levels of apo-enzyme that prevail in tissue could be influenced by other metals.