The study of the link between genes controlling protein polymorphism and milk performance traits of domestic animals has great economic importance from a selection point of view, as it reduces the generation interval, leading to increased productivity in livestock. The objective of this paper was to establish the influence of genetic ? -lactoglobulin polymorphism on the quantity and quality of milk of the simmental breed in Serbi?. For the research blood samples were taken from a total of 157 Simmental cows. The genotypes of Simmental cows for ?-lactoblobulin and their effect on quantitative milk performance traits were determined using the PCR-RFLP analysis. The variability of traits influenced by the genetic polymorphism of ?-lactoblobulin was statistically very highly significant (p <0.0001) for milk yield in standard lactation and milk fat and protein yields, while it showed no statistically significant variability (p> 0.05) for content of milk fat and protein. The genotype cows achieved a 121 kg and 338 kg increase in milk production compared to the BB genotype and AA genotype cows, respectively.