[Show abstract][Hide abstract] ABSTRACT: The effect of two Chinese herbs (Astragalus radix and Scutellaria radix) on non-specific immune response of tilapia, Oreochromis niloticus, was investigated. Three-month-old tilapia with an average initial weight of 62.8 ± 5.40 g were fed diets containing four doses of Astragalus radix (0; 0.1; 0.5 and 1.0%) and four doses of Scutellaria radix (0; 0.1; 0.5 and 1.0%) for 4 weeks. Respiratory burst activity (extracellular superoxide anion production), phagocytosis by isolated blood leukocytes (neutrophils and monocytes) and lysozyme activity in plasma were investigated 1, 2, 3 and 4 weeks after feeding. Both Astragalus radix and Scutellaria radix can modulate the innate immune system of tilapia. The results of this study showed that feeding tilapia with 0.1 and 0.5% Astragalus radix for 1 week enhanced lysozyme activity and for 3 weeks stimulated phagocytosis by phagocytic blood cells, however, respiratory burst activity of phagocytic cells was not increased. On the other hand, inhibition of phagocytosis and respiratory burst activity was found when tilapia was fed with medium and high doses of Scutellaria radix, but not lysozyme activity. There were no significant differences among groups fed with different doses of both herbs. The optimal doses of Astragalus were 0.1% and 0.5% with an optimal feeding period of 3 weeks. Further work is needed to establish the stimulatory dose and optimal time of feeding of Scutellaria.