Although antibiotics treatment has been used as an efficient technique to control infectious diseases in poultry industry, there are intensive studies to decrease the use of antibiotics because of the increase in microbial resistance. Therefore, enhancing protective immunity with immunostimulators by magnifying the capacity of immune response seems to be the most promising and practical approach. Alliums have been reported to possess therapeutic properties often attributed to sulphur-containing compounds (Bonaccorsi et al., 2005). In addition to these compounds, Allium cepa is among the richest dietary flavonoids sources; those flavonoids have shown in mammals in vitro antimicrobial and anticarcinogenic activities (Block et al., 1992). However, very few studies aimed at examining their effects in poultry have been performed. The objective of this study is to examine if the addition of supplementary Allium cepa to a balanced ration enhances the immune response of chickens after immunization.