The article presents experimental data on the use of different levels of protein concentrate supplement in the diets of lactating cows with a high proportion of indigestible protein in the rumen. During the course of the research it was observed that the inclusion of Agro-Matic protein concentrate in the amount of 1.0 and 1.5 kg per head per day in the diet instead of other protein feeds increased the level of indigestible (by-pass) protein by 6.3 % and 9.1 %, respectively. The introduction of the maximum amount of Agro-Matic protein concentrate into the diet has contributed to the change in the amino acid composition of milk. It was established that the yield of milk protein during the period of milking was significantly higher in animals receiving different levels of protein concentrate compared to those of the control group. Feeding lactating cows diets with different levels of protein concentrate did not have a negative impact on biochemical parameters of animal blood. All measured blood of experimental animals were within the physiological norms. The maximum content of essential amino acids in the milk of cows from 3 experimental groups was significantly higher and reached up to 1560.9 mg% versus 1480.17 mg% in the control group.