One of the factors governing economically grounded farm stock breeding is high quality of fodder. Special role in upgrading nutritive, energy and biological value of feed resources is attributed to various additives. In this regard, active (living) yeast and their bioactive metabolites are of great interest. It was proved that upon regular consumption they cause a favorable effect on stock and poultry digestion, protect the gut environment when the rations undergo changes, stimulate secretion of peptic enzymes, intensify uptake of nutrients by the mucous lining of the gastrointestinal tract, inhibit development of pathogenic microbiota, normalize rumen and gut biocenosis. The consequence of such impact is the significant decline in risk of provoking acidosis, diarrhea and other diseases related to malfunctioning of vital organs and systems in animal body. It was also reported that yeast-like fungi showing antioxidant capacity, antiinflammatory activity, and immunomodulating potential, improve significantly physical fitness and reproductive health status. Researchers of Institute of Microbiology, NAS of Belarus have developed feed additive Polyact containing consortium of two yeast-like fungi represented by genera Cryptococcus and Rhodotorula and the derived metabolites – poly- and oligosaccharides, carotenoids, enzymes, peptides. It was found that application of new feed product promoted activity of intestinal microbiota in young cattle and poultry, recovered metabolic processes in the body, displayed hepatoprotective effect. Aim of this study was to assess the influence of feed supplement Polyact on immune status of broiler chickens. In this regard, parameters characterizing natural fowl body resistance were investigated: lysozyme and bactericidal activity of serum, phagocytic activity of leukocytes, their phagocytic number and phagocytic index. According to the obtained data, lysozyme activity of chicken serum in all groups varied from 13.76±1.15% to 14.20±1.29% at the startup (initial) phase of the experiment and fell thereafter. However, this blood parameter in broilers of test groups receiving liquid feed additive Polyact at the dose 2 and 3 ml/capita by 14 and 42 days proved significantly (P˂0,05) higher than in broilers of control group. Likewise, statistically significant surplus of serum bactericidal activity was recorded by 28 and 42 days in poultry of both test groups compared with the control group. Phagocytic activity of leukocytes in blood of broilers from test groups checked upon 14 days of Polyact feeding exceeded the similar parameter in chickens from the control group by 8.8-9.6%. This correlation lasted throughout the whole experiment so that by 42 days the excess of phagocytic activity level in the test groups constituted 10.1-10.7%. At the same time, no significant differences in phagocytic number reflecting the average number of microorganisms ingested by one leukocyte, were found between broilers from the control and test groups. Nevertheless, this parameter in poultry from test groups exceeded the control value, indicating better neutralizing capacity of their leukocytes. An extra evidence of enhanced protective capacity of leukocytes under the action of liquid feed additive Polyact introduced into broiler rations is phagocytic index defining the ratio of total number of microorganisms captured by leukocytes to the total number of examined leukocytes. This parameter in test group was shown to be superior relative to the control in the course of the whole experimental period. In conclusion, the elevated levels of bactericidal and lysozyme activity in serum, the increased phagocytic activity of leukocytes, phagocytic number and phagocytic index values result from activation of native resistance and immunological reactivity in broilers consuming feed additive Polyact.