Introduction Antimicrobial peptides, including cathelicidins, play a significant role in farm animals, influencing animal welfare, immunity, and thus the quality of animal products. Material and Methods The study used amplification-created restriction site and PCR-restriction fragment length polymorphism to analyse single nucleotide polymorphisms of the CATHL7 gene encoding the BMAP-34 protein in cattle, at positions 2,383 G > C and 2,468 G > C. The material was collected from 279 Polish Black-and-White Holstein-Friesian dairy cows. Results There were statistically significant differences between milk performance parameters in cows with the CATHL7/HhaI and CATHL7/HinfI genotypes. In the case of the CATHL7/HhaI polymorphism, the highest milk yield and protein and lactose content and the lowest somatic cell count in milk were observed for the CC genotype, while the fat content was the highest in milk from cows with the GG genotype. In the case of the CATHL7/HinfI polymorphism, the highest protein and lactose content in milk was observed for the CC genotype. Conclusion The results were statistically significant, which suggests that the search for relationships can be continued, and that the results can be used to improve selection programmes supporting dairy farming.