Biochemical indicators of copper status were studied in young women housed in a metabolic unit for 135 days. The women were fed a diet low in Cu (0.67 mg/d), that was supplemented with 1.5 g of ascorbic acid and variable amounts of copper. Copper balance was not significantly affected by diet. Plasma copper and monoamine oxidase, and erythrocyte superoxide dismutase were not affected by dietary copper or by ascorbic acid. Enzymatic (CpENZ), but not immunoreactive (CpRID), ceruloplasmin was lower at the end of low copper intake and high ascorbic acid intake periods. The specific activity of ceruloplasmin (CpENZ/CpRID) was significantly lower at the end of the ascorbic acid period (1.26) than at either the end of control (1.81) or repletion (1.80) periods, and slightly lower than after only low copper intake (1.44). Cytochrome-c oxidase activities in platelets and leukocytes were significantly affected by copper intake, but not ascorbic acid intake. These findings indicate that the specific activity of ceruloplasmin and cytochrome-c oxidase activity of platelets and white cells may be sensitive indicators of copper status, and that ascorbic acid supplements do not markedly affect commonly measured indices of copper metabolism, except for the specific activity of ceruloplasmin in young women.