No sensitive functional index is currently available to assess Cu status in healthy human populations. This study evaluated the effect of Cu supplementation on putative indices of Cu status in twelve women and twelve men, aged between 22 and 45 years, who participated in a double-blind placebo controlled crossover study. The study consisted of three 6-week supplementation regimens of 3 mg CuSO4, 3 mg Cu-glycine chelate and 6 mg Cu-glycine chelate, each separated by placebo periods of equal length. Women had significantly higher caeruloplasmin oxidase activity (P < 0·001), caeruloplasmin protein concentration (P < 0·05), and serum diamine oxidase activity (P < 0·01) at baseline than men. Erythrocyte and leucocyte superoxide dismutase activity, leucocyte cytochrome c oxidase activity, and erythrocyte glutathione peroxidase activity did not respond to Cu supplementation. Platelet cytochrome c oxidase activity was significantly higher (P < 0·01), after supplementation with 6 mg Cu-glycine chelate in the total group and in women but did not change in men. Caeruloplasmin oxidase activity was significantly higher (P < 0·05), in men after supplementation with 3 mg Cu-glycine chelate, while caeruloplasmin protein concentration was significantly lower in men after supplementation with 6 mg Cu-glycine chelate (P < 0·05). Serum diamine oxidase activity was significantly higher after all supplementation regimens in the total group and in both men and women (P < 0·01). These results indicate that serum diamine oxidase activity is sensitive to changes in dietary Cu intakes and may also have the potential to evaluate changes in Cu status in healthy adult human subjects.