SummaryThe study was performed to evaluate the effect of a dietary level of two types of inulin differing in the degree of polymerization (DP), supplemented at different levels, on selected metabolic and immunological parameters of broiler chicken blood. Two hundred and forty 1-day-old broiler chickens were fed a diet without inulin addition (control group, C) or with standard inulin from chicory root with DP ≥ 10 (SI) or long-chain inulin (LCI) of DP ≥ 23 (Inulin Orafti ® GR or Inulin Orafti ® HPX, respectively; Orafti Beneo GmbH, Mannheim, Germany) at a level of 0.2%, 0.4%, or 0.6%. Therefore, 7 dietary treatments were formed. The experiment was carried out for 6 weeks. The addition of inulin had a significant (p ≤ 0.05) impact on the blood parameters analyzed, especially on the protein and lipid profile. The degree of polymerization of inulin and its level in the diet significantly affected the content of glucose and uric acid and creatinine levels in the blood plasma of 21-day-old chickens. The addition of the different types (SI vs. LCI) and levels (0.2, 0.4, and 0.6) of inulin to the diet and the interaction of these factors affected the total protein level and the content of albumins and globulins. Some differences (p ≤ 0.05) were found between the experimental groups in total cholesterol and its HDL fraction content as well as in the activity of ALT and LDH. The other biochemical indices were not affected by the experimental factors. In conclusion, it may be stated that inulin with the higher polymerization degree (LCI; DP ≥ 23) provided better results of the blood metabolic profile throughout the broiler fattening period. However, the impact of this factor is not explicit. The addition of the inulin extract at an amount of 4-6 g per kg of mixture is recommended, but further experiments are recommended.