The expediency of using the probiotic drug Biolax-U in the rations of lactating and dry cows of Black-and-White breed has been studied and scientifically proved. The influence of this drug on the morphological and biochemical parameters of animal blood has been determined. The composition of the probiotic drug Biolax-U consists of eight strains of a balanced complex of living lactic acid bacteria, yeast and actinomycetes, as well as biologically active substances. For experimental studies conducted on 2 groups of cows have been selected by the method of pairs-analogs, the influence of feeding of feed have been prepared with the use of the microbiological drug with probiotic action Biolax-U on feed consumption, morphological and biochemical changes in blood has been studied. Analysis of the rations of cows indicates that the content of calcium, phosphorus, magnesium, sulfur and trace elements such as manganese, copper, zinc and cobalt in the feed has been reduced. It has been found by studies that the use of probiotic drug Biolax-U had the positive influence on the feed intake, digestibility and better assimilation of feed nutrients and contributed to the improvement of blood morphological and biochemical parameters. For example, in animals of the experimental group hemoglobin has increased by 3,6 % (Р < 0,05), red blood cells by 10,1 (P ≤ 0,05), the content of leukocytes by 4,43, carotene by 9,6, glucose by 3,4; reserve alkalinity by 15,6, calcium by 19,4 (P ≤ 0,01), and phosphorus by 18,4 % compared with the control group of animals. During the lactation significantly improved hematological parameters including alkalinity reserve by 5,11 %, calcium level by 11,1, phosphorus by 8,6, hemoglobin by 14,2 %, the content of erythrocytes by 12,3, leukocyte by 3,86, and carotene 11,1 % against the parameters of animals of control group. The improvement of morphological and biochemical parameters of blood of cows indicates about stimulating effect on hematopoiesis and the general clinical condition of animals that contributes to the improvement of metabolic processes when using feed prepared with the use of microbiological drug with probiotic action Biolax-U.