Respiratory burst, lysozyme, phagocytic activities, and immune gene expression levels were assessed in olive flounder (Paralichthys olivaceus, 10.52±2.5 g) fed with 0%, 0.025%, 0.05%, 0.1%, 0.2%, and 0.4% Lonicera japonica leaf powder (LjLP) for four weeks. All flounder fish fed LjLP showed significantly increased respiratory burst, lysozyme and phagocytic activity. Total immunoglobulin levels in LjLP diet fed fish groups were higher than in fish fed the basal diet (BD). In flounder fed LjLP, immune gene expression and antioxidant activity were significantly enhanced after 4 weeks of culture. One week following a challenge with Vibrio anguillarum, fish fed with different concentrations of LjLP showed decreased cumulative mortality and enhanced immunity all compared to the BD diet group. Our findings have shown that Lonicera japonica leaf powder enhanced immune response and resistance to V. anguillarum infection in olive flounder
