W. 2017. Effect of dietary supplementation with arginine on haematological indices, serum chemistry, carcass yield, gut microflora, and lymphoid organs of growing turkeys. A 8-wk feeding experiment was conducted to investigate the effect of dietary supplementation 25 with Arg on haematological indices, serum chemistry, carcass yield, gut microflora, and 26 lymphoid organ weights of growing turkeys. A total of one hundred and eighty 56-d-old male 27 grower turkeys were weighed individually and randomly assigned to 1 of 3 dietary treatments 28 with 6 replicate pens, and 10 turkeys per pen in a completely randomized design. Dietary 29 treatments consisted of basal diets supplemented with 0, 0.5, and 1.0 g Arg/kg. Haematological 30 indices and serum chemistry were measured at 84 and 112 d of study. Carcass yield, relative 31 weights of retail cuts, organ weights, and gut microflora were measured at d 112. Except 32 eosinophil, no effect of Arg supplementation was obtained on haematological indices at d 84. At 33 d 112, finisher turkeys fed the diet supplemented with 0.5 g Arg/kg had the greatest red blood 34 cell (quadratic, P < 0.001), lymphocyte (linear, P = 0.011; quadratic, P < 0.001), and basophil 35 counts (quadratic, P < 0.001). In grower turkeys at d 84, total serum protein (quadratic, P = 36 0.030), and serum globulin concentrations (quadratic, P = 0.043) increased initially as Arg 37 supplementation increased from 0 to 0.5 g/kg, but decreased with the 1.0 g Arg/kg. Uric acid 38 concentration and alanine aminotransferase activity reduced as Arg supplementation increased 39 from 0 to 0.5 g/kg, but increased with the 1.0 g Arg/kg (quadratic, P = 0.002). In finisher turkeys 40 at d 112, total serum protein (linear, P = 0.004; quadratic, P = 0.002), serum globulin (linear, P = 41 0.008; quadratic, P = 0.030), serum albumin (linear, P = 0.012; quadratic, P = 0.040), and 42 triodosterine concentrations (linear, P = 0.025; quadratic, P = 0.033) increased with increasing 43 Arg supplementation. At d 112, spleen weights increased linearly (P = 0.006), while thymus 44 weights increased quadratically (P = 0.003) with increasing dietary Arg supplementation. 45 Salmonella counts in the small intestinal content of turkeys at d 112 reduced quadratically as Arg 46 supplementation increased from 0 to 1.0 g/kg (P = 0.029). In conclusion, Arg supplementation 47 increased packed cell volume of finisher turkeys, improved serum chemistry of grower, and 48 finisher turkeys as indicated by increased total serum protein, and reduced serum enzymes with 49 appreciable improvement obtained when included at 0.5 g Arg/kg. Arginine supplementation 50 enhanced the relative weights of thymus, spleen, and reduced Salmonella counts in small 51 intestine of turkeys.52