The experiment was performed on 128 Black-and-White cows and 121 Jersey cows. The aim of the study was to analyse the milk performance of primiparas of both breeds during 305 days of milking and over the full lactation period, taking into account the polymorphism of milk proteins, as well as to determine the chemical composition of milk at particular stages of lactation, on the basis of selected parameters. The occurrence of a rare casein variant CSN2 (BB) was recorded in Jersey cows. It was found that in both breeds the A gene variant was more common than the B gene variant. The milk performance of homozygotic primiparas (AA and BB) was higher than the milk performance of heterozygotes. Milk production was higher over the full lactation than during 305 days of milking, especially in terms of the ECM yield. The milk yield was increasing for the first 60 days of lactation, and then decreased. The concentrations of milk protein and milk fat exhibited the opposite tendency, i.e. reached the lowest level at 60 days of lactation and then increased. The levels of dry matter and casein were increasing over the lactation period.