In view of the threat to local breeds resulting from intensive animal production, many studies are conducted in search of arguments confirming their importance in food production. In the case of milk production, not only is its quantity important, but its quality is as well, including its chemical composition. Particular focus has recently been placed on the casein proteins beta-casein (CSN2) and kappa-casein (CSN3), due to their potential impact on human health or on the suitability of milk for cheese production. The present study analysed the polymorphism of these proteins in 1777 cows belonging to two local cattle breeds, Polish Red and Polish White-Backed, using Illumina Infinium XT SNP technology on a EuroGenomics MD chip. The results indicate that the Polish White-Backed breed is predisposed to produce ‘A2 milk’, as the frequency of the CSN2 A2 allele in the population was 61.2%. The Polish Red breed was characterised by a higher frequency of the CNS3 B allele (35%), which according to extensive scientific literature is associated with better coagulation properties, and increased whey expulsion. The highest yield of milk and its constituents, confirmed at p ≤ 0.01, was obtained for Polish White-Backed cows with the A2A2 genotype in CSN2 and cows with the AA genotype in CSN3. In the Polish Red breed, no statistically significant differences were obtained between means for milk production traits.