The authors found that PRL promoted an inflammatory reaction in the epithelial cells of the mammary gland of cattle through the activity of nuclear factor kappa B. We determined that in groups of cows diagnosed with mastitis, number of animals with bPRL-RsaIBB associated with increased milk productivity exceeded the theoretically calculated equilibrium value, which may indicate that animals with increased milk productivity are more susceptible to mastitis. Here, PRL is linked to mastitis.