The aim of the present study was to analyze the yield and contents of milk fat and milk protein, and to determine the correlations between these parameters on the basis of 5471 lactations, including 1955 lactations after the first calving, 1228 after the second calving, 914 after the third calving and 1374 after more than three calvings of Black-and-White cows (with a higher protein content of milk than in the domestic population), taking into account the milk protein to milk fat ratio. Depending on this ratio the cows were divided into three groups, i.e. <0.8, 0.8-0.9, and >0.9. Significant correlations were observed between the contents of milk fat and milk protein in these groups. It was found that an increase in the protein to fat ratio from <0.8 to 1 is accompanied in successive lactations by a decrease in the milk yield and the contents of fat and protein. This indicates that it is difficult to maintain high productivity of cows and a well-balanced protein to fat ratio.

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