The aim of this study was to evaluate the effects of graded Cu and Zn on immune system. A total of 360 day-old broilers were distributed into 9 treatments with 4 replicates using CRD design (3x3 factorial). Chicks for 42 days received Cu (35, 70, 105 mg kg-1 diet) and Zn (40, 80, 120 mg kg-1 diet). On days 8, 12 and 15, chicks were vaccinated against Newcastle Disease Virus (NDV), Infectious Bronchitis Virus (IBV) and Infectious Bursal Disease (IBD), respectively. Two chicks from each pen were bled on days 6 and 12 after vaccinations and collected sera were used in Hemaglutination Inhibition Test for NDV and ELISA test for IBV and IBD. On day 42, one bird from each pen was bled for counting leukocytes. Data analysis showed no significant interaction between graded Cu and zinc. The main effect means of titers increased by Cu at 105 ppm for NDV (day 12), IBV (days 6 and 12) and IBD (days 6 and 12). Zinc at 120 ppm slightly improved titers of NDV (day 6), IBV (day 6) and IBD (day 12). Graded Cu significantly enhanced number of heterophils and ratio of heterophil to lymphocyte (H:L), but reduced number of lymphocytes (p<0.05). Conversely, graded Zn significantly decreased H:L (p<0.05), but increased lymphocytes and monocytes and decreased heterophils. In conclusion, supplementations of Cu and Zn promoted immune system in broilers.

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