Plasma Cu concentrations and ceruloplasmin (a Cu metalloenzyme) activity were used to assess relative Cu bioavailability from different Cu sources for growing cattle. In Exp. 1, 18 calves (average BW 207 ± 7.7 kg) that had been fed Cu-deficient diets since birth were randomly assigned to treatment. Treatments consisted of control (n = 3) or 30 mg/d of supplemental Cu (n = 5/treatment) from Cu oxide (CuO), Cu sulfate (CuSO4), or Cu lysine. Blood samples were obtained for plasma Cu and ceruloplasmin activity on d 0, 7, 14, and 21. Plasma Cu and ceruloplasmin activity were greater (P < .05) on d 7, 14, and 21 for calves supplemented with CuSO4 than for controls. Copper status of calves fed Cu lysine did not differ from that of calves fed CuSO4. Compared with d-0 values, plasma Cu concentrations by d 21 had increased by 95 and 98% in calves supplemented with CuSO4 and Cu lysine, respectively. Copper oxide supplementation did not increase Cu status above that observed in control calves. In Exp. 2, 36 calves (average BW 211 + 4.4 kg) were used to compare the relative bioavailability of Cu from CuO and CuSO4 when supplemented to corn silage-based diets high in the Cu antagonists iron (Fe) or molybdenum (Mo). Treatments consisted of no supplemental Cu or 8 mg of supplemental Cu/kg diet from either CuSO4 or CuO. Within each Cu treatment, diets were supplemented with 600 mg of Fe or 5 mg of Mo/ kg diet. In calves fed 5 mg of Mo/kg, plasma Cu was lower (P < .05) in those fed no supplemental Cu or CuO compared with calves fed CuSO4 by d 91 and at subsequent sampling dates throughout the 154-d study. Plasma ceruloplasmin activity was decreased (P < .01) by Mo and was increased (P < .05) by CuSO4 compared with CuO. Based on plasma Cu and ceruloplasmin activity, CuSO4 and Cu lysine were similar in bioavailability, but CuO was essentially unavailable.