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. In heifers at an early age, the level of the enzyme AST in the blood serum can be successfully predicted productivity in the adult state. The terms of use of productive animals directly depend on the level of transaminase concentration in young animals aged 9‒12 months. A new classification of performance types has been formulated, as well as an analysis of the nature of inheritance of milk productivity in the interior-complementary selection of animals has been given