OBJECTIVE In recent decades, the use of various feed supplements is the current trend in poultry farming, among which phytogenics serve as alternatives to feed antibiotics. This study aimed to examine the effect of feeding various doses of milk thistle extract (Silybum marianum) on the morphological and biochemical parameters of the blood in broiler chickens. METHODS Experiments were carried out in an industrial poultry farm on broiler chickens of the Hubbard ISA F15 cross for 40 days. One control group and five experimental groups of day-old chickens were formed. The number of birds in each group was 50. Broilers of all groups received complete feed, and the experimental groups received an additional milk thistle extract at doses of 0.1, 0.5, 1.0, 1.5, and 2.0 mg/kg of body weight. Milk thistle medicinal plant extract was obtained using water-ethanol extraction followed by low-temperature vacuum drying. For the assessment of blood analyses, samples were collected from the wing vein of six chickens per group. Using unified methods recommended by the International Federation of Clinical Chemistry, the content of red blood cells, hemoglobin, white blood cells, total protein, protein fractions, triglycerides, glucose, calcium, phosphorus, and the concentration of alanine aminotransferase and aspartate aminotransferase in the blood serum were determined. RESULTS It was found that the introduction of milk thistle extract into the diet of broiler chickens with the aforementioned doses increased the number of red blood cells, hemoglobin, white blood cells in the blood, as well as a decrease in the level of albumin and an increase in the content of γ-globulins in its serum. CONCLUSIONS The authors assume that the introduction of milk thistle extract into a complete feed for broiler chickens increased the anabolic processes in their bodies, accompanied by increased use of proteins of the albumin fraction as the main material for organogenesis.