Two independent experiments were conducted with Ross broilers to determine the incremental levels of threonine and vitamin A on immune system. In first experiment one-day-old Ross broilers were randomly assigned to 16 pens with 10 chicks per pen in a completely randomized design. The dietary treatments consisted of an isonitrogenous corn-soybean meal-based diet with the addition of graded levels of L-threonine supplementation. Dietary treatments contained 0.8% (NRC requirement), 0.87% (average of NRC and Ross requirement), 0.94 % (Ross requirement) and 1.01 % (more than Ross requirement) total threonine. In second experiment, one hundred sixty day-old male broiler chicks (Ross 308) were randomly assigned to 16 pens measuring. All birds fed wheatsoybean meal based diets. The diets were formulated to either meet or exceed the recommended NRC (1994) nutrient requirements for broilers. Experimental diets were as follows: basal diet without vitamin A; basal diet plus 1500 IU/Kg vitamin A (NRC 1994 requirement); basal diet plus 6250 IU/Kg vitamin A (between Ross 2007 and NRC 1994 requirement); basal diet plus 11000 IU/Kg vitamin A (Ross 2007 requirement). Fourteen days after feeding the treatments, cutaneous basophil hypersensitivity (CBH), cell-mediated immune response, was determined as the increase in toe-web skin thickness after an injection with phytohemagglutinin-P and twenty-two days after feeding the diets, heterophils and lymphocytes were enumerated. Humoral immunity was assessed by intravenous injection of 7% sheep red blood cell (SRBC) at 22 days of age. In first experiment, the titers of IgG, IgM and IgA for primary and secondary responses were numerically, but not significantly, increased in birds fed diet containing 0.87 % threonine and decreased in 0.94% and 1.01% threonine. Cutaneous basophil hypersensitivity was not affected in chicks, whereas there was a tendency for an increase in birds fed a diet containing 0.87 % threonine at 24 h after injection. In second experiment, the titers of IgG, IgM and IgA for primary and secondary responses were significantly increased in birds fed diet containing vitamin A. cutaneous basophil hypersensitivity was also increased by addition vitamin A in the diet. The results obtained on the present study indicated that threonine and Vitamin A requirements of broiler based on recommendation of NRC are not sufficient to meet the requirement of the new commercial poultry and commercial broiler companies under non-hygienic condition.

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