In the study, the dietary supplemented mistletoe extract appeared to significantly elicit non-specific immune responses in terms of the respiratory burst activity, lysozyme activity, phagocytic activity, and ACH50 activity in tilapia. ROS are produced during the respiratory burst of phagocytes and play an important role in the antimicrobial activity of phagocytic cells [40]. A similar result of enhanced ROS production by biood leucocytes was observed in tilapia fed with a B-glucan supplemented diet [16]. Regardless of the assayed periods, a significant respiratory burst activity was enhanced in all groups fed with different doses of mistletoe compared to the control. On day 20 and 40 days of feeding period, group C (50 mg dry diet) showed the highest level of respiratory burst activity which was almost similar each other between group C and D on 80 days and during the post-challenge period. The result suggests that the respiratory burst activity could be positiveiy induced by an appropriate level of mistletoe. Ai et al. [41] reported that feeding a high dose of B-1, 3 glucan resulted in immunosuppression or feedback regulation in their work. Recentiy, Luo et al. [42] and Geng et al. [17] also supported our result reporting that the high chitosan group showed significantly inhibited respiratory burst activity compared to the low level of chitosan group.