Purpose. To study lactulose-containing prebiotic feed additives in feeding Kalmyk fat-tailed rams, evaluate their effect on growth and development indicators, and determine the physiological state of animals when they are used. Materials and Methods. LLC "Baska" of Yustinsky district of the Republic of Kalmykia was chosen for the experiment. The experimental part of the work was conducted on 45 rams of Kalmyk fat-tailed breed, selected according to the principle of pair-analogs in three groups of 15 animals each. The animals in the first group (control group) received general economic ration (OR); the second group – OR + feed additive "Lactumin-1" at a dose of 0.6% of the weight of concentrates; the third group – OR + feed additive "LactuSuper" at a dose of 0.5% of the weight of concentrates. The conditions of the experimental young animals were the same for all experimental groups and corresponded to the generally accepted zootechnical and veterinary norm. The study of blood parameters was carried out in the conditions of Complex Analytical Laboratory of VRIMMP (Volgograd) using analyzers URiT-3020 Vet Plus (China) and URiT-800 Vet (China), lysozyme, bactericidal and phagocytic activity – using the methods of Dorofeychuk V.G. and Fedyuk V.V. In order to study the effect of lactulose-containing feed additives on the live weight of experimental animals, the rams were individually weighed every 30-31 days. The results obtained during the experiment and laboratory conditions were processed using mathematical and statistical methods by calculating the reliability criterion (Student-Fisher method) in the Statistica 10.0 program. Results. Addition of lactulose-containing additives to the diet had a positive effect on blood parameters, contributed to the improvement of the natural immunity of animals and an increase in live weight: at the age of 4 months the content of total protein in the rams blood of I and II groups increased by 1.63% (P≥0.95) compared to the control group, lysozyme activity – by 1.4 and 2.7% (P≥0.95), bactericidal – by 0.37 and 1.24%, phagocytic – by 3.39% (P≥0.95) and 4.7% (P≥0.99), live weight – by 4.93% (P≥0 .99) 7.56% (P≥0.999); at the age of 6 months the amount of total protein was greater by 1.63 and 3.29% (P≥0.95), lysozyme activity – by 1.88 and 2.84% (P≥0.95), bactericidal activity – by 0.59 and 1 .42%, phagocytic – by 1.54% (P≥0.95) and 3.98% (P≥0.99), live weight – by 2.39% (P≥0.95) 4.36% (P≥0.99), respectively. Conclusion. Lactulose-containing feed additives had a beneficial effect on blood biochemical parameters, increased the level of natural immunity of animals, and contributed to an increase in meat productivity of rams.