The purpose of the research is to study the probiotic Subalin effects on body weight gain in broiler chickens with spontaneous eimeriosis.Materials and methods. On a poultry farm in the Moscow Region, the body weight gain dynamics was studied in 120 floormanaged broiler chickens aged 9 to 40 days spontaneously infected with eimeria after administration of different doses of the probiotic Subalin vs. control. The chickens selected for the experiment were divided into four similar groups of 30 birds each. Subalin was administered to the first group chickens at a dose of 1.5 mL per 1 L of drinking water, while the broilers of the second and third groups were administered Subalin at a dose of 2 and 2.5 mL per 1 L of water, respectively. Subalin doses were given to the chickens of groups 1–3 in three stages: on 11–12; 18–19 and 26–27 days of age for consecutive 48 hours. The broilers of group 4 did not receive the drug and served as a control. The chickens of all groups underwent clinical, hematological, and coprological examinations; they were kept in similar conditions on the floor, in isolation, and fed as per zootechnical standards. Control weighing of the chickens was done on days 10 and 40 of their growth.Results and discussion. The total weight gain of the group 1 chickens was 1985 g in the experiment; 2203 g and 2214 g in groups 2 and 3, respectively, and 1803 g in the control group. The productivity of the experimental chickens was 10.1; 22.2 and 22.8% higher vs. control. The Subalin optimal dose was 2 mL per 1 L of drinking water according to the above scheme. The tested doses of Subalin had positive effects on the number of enterobacteria in the chicken intestines. Blood tests showed no statistically significant changes in clinical or hematological parameters in the experimental and control chickens.