Анотация: In this report a persistently infected cow was under surveillance for three years. In that period the level of antibodies and level of viremia were determined. Persistent infection was confi rmed in blood serum samples taken at 3 week intervals and tested on the virus antigen by antigen enzyme-linked immunosorbent assay and by inoculation in cell culture, which after incubation was stained by an unoperoxidase technique. In order to determine the changes in antibody level and virus titer levels, blood samples were collected and tested each six months. Also, data on milk production were collected. The level of viremia was 10 4.5 throughout the period of the study, and antibodies were not detected by VNT. Milk production was above-average at all times. Average daily milk production over 305 days of lactation in 2009 and 2010 was 25 and 21 litres per day respectively. In the fi rst fi ve months of 2011 daily milk production was 15 litres per day. The cow was taken out of herd at the age of seven years