The investigations were carried out on 52 495 cows of Black-and-White and Holstein-Fresian crossbreds, belonging to an active population on the premises of former provinces in Poland: Bydgoszcz, Toruń and Włocławek. Calving occurred over 1999 and 2000 (lactations ≥ 200-days). On the ground of independence test results (χ²) one was observed significant differentiation frequency of cows with definite difference between fat and protein content in their milk. On the average, cow proportion in milk of which protein content is equal or higher than fat was 1.07%, and their proportion grew larger together with the following lactations, with progress of grading crossing by Holstein-Fresian race, and also with herd number. The opposite situation was in the case of cow frequency with unbeneficial difference between fat and protein content in milk from the breeding view-point, because it exceeded 1%. Difference value between fat and protein content differentiated to advantage of ”protein cows”, milk and protein yield for lactation, protein to fat ratio.

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