The objective of the research conducted was to determine the effect of cow breed on the chemical composition and content of some selected whey proteins in milk and in rennet whey. The research comprised 2278 milk samples in total, i.e. 789 samples of milk from the Black and White cow variety of the Polish Holstein-Friesian breed, 486 from the Red and White cow variety of the Polish Holstein-Friesian breed, 768 from the Simmental, and 235 from the Jersey cows. Analyzed was the basic chemical composition of milk (i.e. the contents of total protein, casein, fat, lactose, and dry matter), as well as of the selected whey proteins (α-lactalbumin, β-lactoglobulin, lactoferrin, bovine albumin serum, and lysozyme). It was found that the breed had a significant effect on the content of basic chemical compounds and on the content of selected whey proteins. The Jersey cows produced milk with the contents of the components analyzed being significantly (p ≤ 0.01) above average. The whey from milk of that cow breed was also characterized by the highest content of total protein including casein. However, the milk and whey from Simmental cows had significantly more whey proteins. It was shown that the content of dry matter in the whey was more than 50 % lower compared to milk, and it also contained trace quantities of fat and the casein content therein was ca. 70 % lower.