№ 2 - 2014 вирулентностью вируса ИРТ, проявляющейся максимальной серопозитивностью коров и высокими титрами антител. <...> SUMMARY The article contains data on manifestation of epizootic process at mixed infection caused by infectious bovine rhinotracheitis (IBR) virus and parainfluenza 3 (PI 3) virus in cows in the conditions of a dairy farm. <...> Goal of investigations was to reveal some predisposing factors for occurrence and manifestation of mixed infection caused by IBR virus and PI 3 virus. <...> During the epizooty 270 cows of black white breed were under surveillance. <...> It was established that predisposing factors for occurrence of mixed infection caused by IBR virus and PI 3 virus in cows were poor nutrition and sharp change in diet. <...> Clinical manifestations of mixed infection caused by IBR virus and PI 3 virus in cows were apathy, decrease of palatability of the feed by 40% and of daily milk yield by 30%, hyperthermia (in 42% of animals), hypersecretion of mucus (in 70% of animals), cough and dyspnea (in 50,4% of animals), mastitis (in 27,2% of animals), abortion (in 12,5% of calved cows), stillbirth (in 8,3% of calved cows), postparturient paresis (in 14% of caved cows), retained placenta (in 22% of calved cows), endometritis (in 36% of caved cows). <...> At the beginning of the development of the epizootic process the seropositivity against IBR virus came to 66,7% at the average antibody titer 1,35 lg2, the seropositivity against IBR virus in conditionally healthy animals was 25% at the average antibody titer 0,67 <...>