Phagocytosis and pathogen killing ability by phagocytic cells is an important defence mechanism in fish against pathogenic organism (Neumann et al., 2001; Rao et al., 2006). The phagocytes in fish are able to produce superoxide anion (O2-) and its reactive derivatives during the period of intense oxygen consumption called the respiratory burst (Secombes and Fletcher, 1992; Secombes, 1996; Sahu et al., 2007). It is also considered that these oxygen forms are toxic to the bacterial pathogens (Hardie et al., 1996; Sahu et al., 2007) and are generated by phagocytes after stimulation. In the present study, the respiratory burst activity of phagocytes was quantified by the nitroblue tetrazolium (NBT) assay, which measures the quantity of intracellular superoxide radicals produced by leukocytes (Siwicki and Studnicka, 2006). A significant increase in respiratory burst (NBT) activity was observed in all the treatment groups fed with tree leaf (Psidium guajava and Mangifera indica) extracts which indicate that the extracts enhanced the formation of reactive oxygen species to fight against the microbial pathogen and stimulate the non-specific immune function compared to the control. This is in line with the work of Jian and Wu (2003, 2004), who reported a significantly increased respiratory burst activity in Jian carp and yellow croaker fed for 20 days with 1% and 1.5% of TCM (mixture of Astragalus; Astragalus membranaceus and Astragalus sinensis) extracts. Similarly feeding of rohu (Labeo rohita) with mango seed kernel increased the superoxide anion production of the blood leucocytes (Sahu et al., 2007a). Also dietary administration of I-carrageenan and azadirachtin were reported to significantly enhance respiratory burst activities in La... ... middle of paper ... ...hat Labeo rohita fed with dietary andrographolide (EC 50%) showed significantly enhanced resistance against A. hydrophila infection. Ardo et al. (2008) and Pachanawan et al. (2008) also reported an increased survival in Oreochromis niloticus fed with Chinese medicine and Psidium guajava respectively, after experimentally infected with Aeromonas hydrophila. In conclusion, it can be inferred that the dietary tree leaf extracts fed in the experiment enhanced the haematological parameters and stimulate the non-specific immunity of Labeo rohita, with better enhancement at 0.5% inclusion level of either mango or guava leaf extracts, and protected the fish when challenged with Aeromonas hydrophila. However, synergistic effect of both the tree leaf extracts may give better result than reported in this study when added at lower inclusion level, but needs further research.