It has been studied how MP-15 and MP-7 preparations fed on to lactating cows affect the indices of acid-base equilibrium of their blood, nitrogen metabolism, milk yield and milk quality. It is shown that feeding on MP-15 and MP-7 preparations to cows of test groups has no negative effect on the acid-base equilibrium indices, decreases the ammonia concentration by 38.5 and 54.9%, respectively, increases the glutamine content in blood by 23.5 and 29.4%, respectively, does not affect the pyruvate, glutamate, oxaloacetate concentration and alanine-transaminase activity, increases the average daily milk yields by 2.1 and 2.3 l, respectively, and does not deteriorate the composition and physicochemical indices of milk quality.