Two genetically related sheep that produced border disease-affected lambs from successive pregnancies were identified. These sheep, and some of their progeny, were found to be persistently infected with a virus antigenically related to bovine virus diarrhoea/mucosal disease virus. Only one of the sheep developed detectable serum antibody to the virus, and this animal only produced it 12 months after being detected as infected. The epidemiological significance of sheep persistently infected with border disease virus is discussed.