Aims: Dietary acidifiers appear to be a possible alternative to feed antibiotics in order to improve performance of broilers. It is generally known that dietary acidifiers lower gastric pH, resulting in increased activity of proteolytic enzymes, improved protein digestibility and inhibiting the proliferation of pathogenic bacteria in GI tract. The present paper assesses the different dosage of an acidifier on commercial broilers. Study Design: Two hundred and Forty day-old chicks were randomly distributed in a completely randomized experimental design with four treatments and three replications of twenty chicks each. Diets prepared without additive as Control (CON) (group1); 0.025% Acidifier Agent (AA1) (group2); 0.05% Acidifier Agent (AA2) (group3) and 0.1% Acidifier Agent (AA3) (group4). Place and Duration of Study: Department of Animal Science, Malayer University, Malayer, Iran, between May 2013 and September 2013. Methodology: At the end of the trials, six birds from each replicate were sacrificed by cutting the jugular vein and blood samples were individually collected in 10-mL heparinized tubes and stored on ice for hematological analysis. Serum was separated after 8 to 10 hours and was stored at –20ºC for subsequent analysis. The individual serum samples were analyzed for antibody titers against Newcastle disease (ND), Infectious Bursal
