The experiment was performed on 121 Jersey cows in the years 2003/2004. The aim of this study was to determine the yields of ECM, milk fat and milk protein as well as milk fat content and milk protein content during four successive 305-day lactations as dependent on the genotypes and polymorphic variants of milk proteins. Depending on their genotype, the cows were divided into the following groups: AA, AB, BB and BC. Four major milk proteins were obtained as a result of separation, i.e. LGB, CSN1S1, CSN2 and CSN3. The yields of ECM, milk fat and milk protein increased during four successive 305-day lactations. Cows with the polymorphic fractions LGB AA and AB, CSN1S1 BC, CSN2 AB and CSN3 AA were characterised by the highest productivity. The highest increase in fat content (0.63%) was recorded between the first and second lactation. The LGB AA genotype contributed to an increase in protein concentration, by 0.14%, 0.12% and 0.19% in the second, third and fourth lactation, respectively. In the case of the other fractions these values were as follows: CSN1S1 BC - 0.13%, 0.07% and 0.11% in the second, third and fourth lactation, respectively, CSN2 AA - 0.09%, 0.20%, 0.08% and 0.04% in the first, second, third and fourth lactation, respectively, and CSN3 BB - 0.11%, 0.16% and 0.10% in the second, third and fourth lactation, respectively.