The aim of this study was to determine whether lambs persistently infected with Border Disease Vi-rus (BDV) can infect seronegative calves and cause transient viraemia and seroconversion, when kept in close contact. After a quarantine period of four weeks, two lambs persistently infected with Border disease virus were introduced into a group of nine calves. The calves were negative for pestivirus-specific RNA and BDV and bovine viral diarrhoea virus (BVDV) serum antibodies. All animals underwent a thorough clinical examination once daily throughout the study. Blood samples were taken for haematological analysis and detection of pestivirus-specific RNA and antibodies every one to two days for 73 days. In addition, nasal swabs were taken for the detection of pestivirus-specific RNA at the time of blood collection. Pestivirus-specific RNA could not be detected in blood samples or nasal swabs in the calves. However, seroconversion had occurred in six of the nine calves 72 days after introduction of the infected lambs. A serum neutralisation test clearly revealed that the antibodies were directed against BDV and not BVDV. It can therefore be concluded that the calves had become infected with BDV. However, the exact time point of infection could not be determined. The results indicate that after completion of the current BVDV-eradication program, sheep persistently infected with Border disease virus may pose a risk for the cattle population.