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BACKGROUND The aim of this study was to investigate whether leucine (Leu) alleviates oxidative injury in bovine intestinal epithelial cells (BIECs) induced by hydrogen peroxide (H2 O2 ) and the underlying molecular mechanisms. RESULTS BIECs were treated with H2 O2 (1 mmol/L) and/or Leu (0, 0.9, 1.8 or 3.6 mmol/L) for 2 h. Leu increased cell viability (p < 0.05) and decreased the release of lactate dehydrogenase (p < 0.05) in BIECs challenged by H2 O2 . Then, the cells were treated with H2 O2 (1 mmol/L) and/or Leu (1.8 mmol/L) for 2 h. Compared with the H2 O2 group, cells treated with Leu and Leu + H2 O2 exhibited increased (p < 0.05) mRNA and protein expression of superoxide dismutase 2 (SOD2), catalase (CAT), glutathione peroxidase 1 (GPx1), heme oxygenase 1 (HO-1) and nuclear factor erythroid 2-related factor 2 (Nrf2). BIECs treatment with Leu significantly reduced (p < 0.05) apoptosis induced by H2 O2 . BIECs were transfected with Nrf2 siRNA for 48 h and/or treated with H2 O2 (1 mmol/L) and/or Leu (1.8 mmol/L) for another 2 h. Transfection with Nrf2 siRNA abrogated the protective effect of Leu against H2 O2 -induced apoptosis and the mRNA and protein expression of SOD2 (p < 0.05). CONCLUSION These results indicate that Leu promotes the relative expression of antioxidant enzymes (SOD2, CAT and GPx1) and phase II detoxification enzymes (HO-1) by upregulating nuclear Nrf2 and activating the Nrf2 signaling pathway, thus enhancing the antioxidant capacity of cells. This article is protected by copyright. All rights reserved.