This study was conducted to evaluate the effect of dietary genistein (Gn) on broiler immune system. A cornsoybean meal basal diet was supplemented with five genistein levels (10, 20, 40, 80 and 160 mg of Gn/kg) in a completely randomised design with 5 replicates of10 birds. Dietary control treatments were included the negative control diet with no feed additive supplementation and two positive control groups supplemented with either virginiamycin or zinc bacitracin. Three hundred and fifty day-old male broiler chicks were fed with the experimental diets from 1 to 42 days of age. On days 13 and 23, chicks were vaccinated against Newcastle disease virus (NDV) and infectious bronchitis virus disease (IBV), respectively. The 10 chicks from each dietary treatment (two per pen replicate) were bled on day 6 and 12 after vaccinations. Sera samples were use in hemagglutination inhibition test for NDV and ELISA test for IBV. Dietary supplementation with 20 mg/kg Gn caused increases (pB0.01) in antibody titres against Newcastle and infectious bronchitis diseases viruses at day 12 after vaccine administration. However, diet modifications had no significant effect on blood leukocyte sub-populations and heterophil to lymphocyte ratio. The present results suggest that dietary supplementation with genistein especially at the levels of20 and 40 mg/kg can improve immunological responses of broiler chicks.

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