The developed method for the prevention of mastitis in lactating cows by intramuscular injection of Biferon-B at doses of 5.0–10.0–20.0 ml, provided a preventive effect in 25.0–75.0 % of animals; the best effect was achieved with Biferon-B at a dose of 5.0 ml. The use of Biferon-B in clinically healthy lactating animals was accompanied by a decrease in blood levels of stab neutrophils by 58.8–65.0 %, circulating immune complexes by 23.4–62.6 %, with a higher content of segmented neutrophils by 4.8–7.8 %, monocytes by 5.6–57.1 %, lymphocytes by 4.9–12.3 %, total immunoglobulins by 5.7–14.3 %, bactericidal and lysozyme activity of blood serum by 6.4–23.1 % and 4.3–13.7 %, respectively. The phagocytic activity of neutrophils decreased by 5.6–10.9 %. With lower indicators of the intoxication index dropped by 12.8–19.1 %, the content of average molecular peptides by 2.4–22.6 %, nitric oxide by 24.5–45.4 %, MDA by 2.8–36.7 %; catalase activity increased by 2.7–12.8 % and GPx by 10.4–29.7 %.