A 2×4 factorial arrangement of treatments was used to investigate the effects of 2 levels of an acidifier supplement (0 or 350 mL/1000 L) in drinking water on growth performance, carcass, immune response, intestinal microbial flora, and jejunum morphology of broiler chickens fed diets based on 4 levels of dietary Thr (100, 110, 120, and 130% of the requirements). A total of 320 broiler chickens were assigned to 8 treatments with 4 replicate pens of 10 broiler chickens per pen. Growth performance traits, including daily weight gain, feed intake, and feed conversion ratio (FCR), were recorded. The broiler chickens immunized against Newcastle disease virus (NDV) at 8 d of age. Blood samples were drawn from the wing vein 7 and 14 d after vaccination for the determination of primary and secondary antibody responses. In addition, blood samples were collected in tubes containing anticoagulant to determine the number of heterophil (H) and lymphocyte (L). At the end of the experiment, 4 broiler chickens per treatment were selected and killed, and, then, the relative weights of carcass parts, jejunum morphology, and intestinal microbial population were determined. The results showed that broiler chickens received acidified drinking water (ADW) had greater (P