Unbalanced nutrition as well as diseases of infectious and non-infectious etiology lead to a decrease in production and a significant loss of farm animals and birds. To mitigate these problems, various probiotic drugs (probiotics) are introduced into the diets of birds and animals for preventive purposes. The aim of the research was to study the effect of the Vitafort probiotic, containing antagonistic bacteria of the endophytic strain Bacillus subtilis B11, on the body of broiler chickens. The effect of Vitafort was assessed by measuring the live weight and average daily growth of birds. In addition, a morphological study of blood was carried out and the content of Bacillus subtilis in the small intestine was determined. It was found that the endophytic strain Bacillus subtilis B11 improved the intestinal microflora. Thus, on day 5 of its use, the number of microorganisms varied from 4.1 × 104 to 8.2 × 104 CFU/g, and on day 42 the same number varied from 4 × 108 to 5.8 × 108 CFU/g on day 42, the contents of red blood cells, haemoglobin, and white blood cells in the blood of chickens from the experimental group were higher by 6.44%, 3.44%, and 2.11%, respectively, than they were in the control group. Considering the above facts, it can be assumed that the absorption of nutrients in the intestine improved, and metabolic processes were activated due to an increase in the transport abilities of red blood cells.