A complete randomized design experiment with a 4 × 4 factorial arrangement consisted of 4 levels of energy (2900, 3000, 3100 and 3200 kcal/ kg) and 4 levels of protein (17, 20, 23 and 26%) was conducted to assess the effect of dietary energy and protein on humoral immune response of chickens. All birds in each replicate were injected intramuscular with SRBC (15% suspension in PBS, 1 mL/chick) at d 15 and 25. Blood samples were collected at d 5 and 10 of injections. The serum from each sample was analyzed for total anti-SRBC antibody as described previously (Cheema et al., 2003). The data were transformed and then analyzed using the GLM procedures of SAS software (v. 9.1). Total anti-SRBC titers were increased in birds fed low energy diet in both postprimary (PPI) and postsecondary injections (PSI), but dietary protein contents did not influence antibody titers of birds. Increasing the level of energy and protein in broiler diets improved growth rate which may cause a negative effect on humoral immune response. Broiler bursa of Fabricius size decreased gradually with increasing dietary energy level, which could affect on immune response. It seems that, there is a negative phenotypic association between immunocompetence and rapid growth in chickens. Table 1. Effect of various levels of ME and CP on total anti-SRBC titers in postprimary (PPI) and postsecondary (PSI) injections Days PPI Days PSI Level 5 10 5 10 ME 2900 5.304a 2.424 6.450 4.894a 3000 4.958b 3.060 6.990 4.559ab 3100 5.048ab 3.555 7.410 4.389b 3200 5.132ab 3.630 6.780 4.096 CP 17 5.341 2.968 7.000 4.372 20 5.233 2.887 6.833 4.508 23 4.929 2.876 7.037 4.639 26 5 3.166 6.775 4.383 Probability ME 0.0152 0.95 0.113 <0.001 CP 0.202 0.145 0.831 0.843 ME×CP 0.766 0.397 0.112 0.179

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