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The aim of this study was to investigate the effect of dietary copper sources and concentrations on serum immune indices and the <em>protegrin-1</em> (NPG1) expression level in the bone marrow of weaning piglets. A total of 80 crossbred piglets (Duroc × Landrace × Yorkshire), with the average age of 21 days and the initial body weight of 7.00±0.03 kg, were randomly assigned to five treatments for 14 days by different kinds of diet. The dietary treatments were: 1, basal diet; 2, basal diet + 20 mg/kg Cu as CuSO4; 3, basal diet + 20 mg/kg Cu as cupric citrate (CuCit); 4, basal diet + 180 mg/kg Cu as CuSO4; 5, basal diet + 180 mg/kg Cu as CuCit. The results showed that compared with basal diet, supplementation with 20 mg/kg Cu as CuCit had no significant difference on growth performance (P&gt;0.05). The incidence of diarrhoea was reduced by 71.57% and serum lysozyme concentration was increased by 170.73% (P0.05). The mRNA expression level of NPG1 was significantly increased by 2.32-fold (P&lt;0.01). However, the other three trial groups showed no significant differences on the experimental results compared with the basal diet group. These results indicated that supplemental 20 mg/kg Cu as CuCit could increase serum lysozyme concentration and NPG1 mRNA expression level, and reduce the incidence of diarrhoea in weaning piglets.