The objective of the research study was to determine the effect of selected milk protein preparations on physicochemical and rheological properties of the fermented milk beverages produced using potentially probiotic Lb. casei bacteria. Milk protein preparations used, in the production of fermented milk beverages, stimulate the growth of the Lb. casei L26 strain. After three weeks of cold storage, the highest survival rate of the Lb. casei L26 strain (3,50 ×10⁸ (cfu/ml) was reported in the products enriched by adding 1% of WPC (whey protein concentrate). The addition of whey protein preparations had an apparent effect on the rheological properties of fermented milk beverages. The curds of fermented milk products produced from skimmed milk were characterized by higher values of the parameters such as hardness or consistency index. The hardness of the curds of fermented milk beverages produced of the skimmed milk powder (OMP) was 0.51 N. The curds of the beverages produced with the addition of 2% of WPI (whey protein isolate) were characterized by a hardness value of 1.65 N. The products produced using the Lb. casei L26 strain were characterized by a high syneresis regardless of the content of fat in milk. The increase of WPI concentration from 0.5 to 2% in the fermented milk beverages made of SMP caused the amount of whey secreted to decrease from 11.98 to 1.03%. In addition to the improved physicochemical properties of the fermented milk beverages, supplementing them with milk protein preparation enhanced the pro-healthy values thereof and gave them the characteristics of functional foods.