The effectiveness of interior-complementary selection of cows by the level of serum enzymes has been revealed. In contrast to traditional selection, the more criteria are taken into account during selection, the more effective it is. When selecting first-calf heifers for one of the enzymes, the difference in milk yield is up to 446 kg of milk, with the total selection for the concentration of serum enzymes, it consistently increases to 656, 1059 and 1228 kg of milk. The same pattern is also characteristic when comparing hereditarily similar animals.