Fig. 1 show the production of ROU from tilapia head kidney cells after feeding mistletoe-treated diet. Significantly (P>0.05) different levels of ROS were observed in groups of fish fed with a mistletoe powder diet of 50 mg and 200 mg through all the assayed periods compared to the control group. The highest superoxide anion production was observed in group C (50 mg) by 40 day of feeding period. Following ten days challenge, groups B, C and D showed significantly higher NBT values compared to the control group (Fig. 1).