Chemical composition of cow`s milk is influenced by a whole series genetic and nongenetic factors. From the environmental factors to the chemical composition of milk, the nutrition, season and microclimatic environment have a particular impact. The correlation of hereditary factors with the chemical composition of cow`s milk began to be particulary explored during the second half of the 20th century, following the discovery of polymorphic forms of certain protein milk. The observed association of allelic polymorphic forms with the cow`s milk, particular the utilization of milk (yield of wheat grain, firmness of wheat grain and other) were the interest of breeding organizations and dairy industry on further investigation of the linkage polymorphic forms of protein with qualitative and processing characteristics of milk. Development of electrophoresis methods, and in particular direct detection of polymorphisms on the responsible coding segments of the gene, stimulated further investigation of the association polymorphism with essential characteristics in the production and processing of the cow`s milk. In thesis deals the association of the beta-lactoglobulin polymorphism with the chemical composition of the Simmental and Holstein cows in the first three lactations production. Two beta-lactoglobulin genotypes (AB i BB) were determined in the investigation population. No significant influence of genotypes and alleles of beta-lactoglobulin on the chemical composition of cow`s milk wasn`t established.