EN
Serological examination of cattle for parainfluenza-3 (PI-3), infectious bovine rhinotracheitis/infectious pustular vulvovaginitis (IBR/IPV), adeno 3 and virus diarrhoea-mucosal disease (VDMD) viruses showed that changes in PI-3 antibody level depended on the age of exam ined animals. After calving, the antibody level decreased significantly in comparison with that observed for a 4-month period before calving. The PI-3 virus antibody level raised as the age of animals increased. This finding suggest that parainfluenza-3 is the natural symptomless infection. It may be activated under conditions favourable to the break-down of non-specific immunity.