The milk yield, the composition and the relationships between the protein and fat content in milk during three lactation periods over 305 days in 1790 Holstein-Friesian black and white cows fathered by bulls from Poland, Holland, Germany, USA and Canada were compared. It was shown that those cows which had been fathered by bulls from the USA showed the highest milk yield during three lactation periods and also significantly the lowest fat content in the milk. They were also characterised by the lowest significant difference between the fat and protein content in the milk (DFP) and the highest relationship of protein to fat in milk (PFR) during three lactation periods. Over three consecutive lactations, the highest protein content was found in the milk of cows which had been fathered by Dutch bulls, while the lowest protein content was to be found in the milk from cows which had been fathered by bulls from the USA. The highest frequency of cows giving milk with a fat content equal to, or lower than, the protein content was found in cows which had themselves been fathered by bulls from the USA; the lowest was in the animals fathered by Canadian bulls. The unfavourable (i.e. the highest) difference between the fat and protein content (DFP) and the least favourable relationship between the protein and fat content (PFR) in the milk, during the three lactation periods, was found in the milk from those cows which had been fathered by Canadian bulls, whereas the best DFP and the most favourable PFR was found in the milk from cows which had been fathered by bulls from the USA.