The objective of this study was to evaluate antibody titers of challenged broiler chicks fed diets supplemented with soy oil. In a completely randomized design, 120 chicks were assigned to 3 treatments (isoenergetic and isonitrogenous diets based on corn-soybean meal containing 0%, 2%, or 4% soy oil), with 4 replicates for each treatment and 10 chicks per replicate. The challenging program included vaccination against Newcastle disease (B1 vaccine on day 12 by eye drop; Lasota vaccine on days

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... and 32 by drinking) and infectious bursal disease (Gumboro D78 vaccine on days 12 and 24 by eye drop). Blood samples were collected from 2 chicks per replicate at days 21 and 42 of age, and then total and differential white blood cells were counted and antibodies against Newcastle and infectious bursal diseases were measured. At days 21 and 42 of age, 2 chicks per replicate were killed and the bursa of Fabricius and spleen were removed and weighed. White blood cell count was the lowest in chicks fed soy oil-free diet. Inclusion of soy oil to diets resulted in significant increase in cell count. Heterophil-to-lymphocyte ratio, as an index of stress, was lower for chicks fed soy oil-free diet than those fed a diet containing soy oil. Relative weights of the bursa of Fabricius and spleen were the highest in chicks fed soy oilfree diet, but they decreased (P < 0.05) as soy oil levels increased in the diet. At days 21 and 42 of age, antibody titers against Newcastle disease and infectious bursal disease viruses were the highest in chicks fed the soy oil-free diet. Antibody titers decreased (P < 0.05) as soy oil levels increased in the diet. It was concluded that the addition of soy oil to the diet resulted in retardation of the immune organs weight, change in the immune cells profile, and consequently suppression of the antibody titers against Newcastle disease and infectious bursal disease viruses in broiler chicks.