Twenty four ewes from a field outbreak of Border disease (BD) in Mule sheep in Scotland and 22 Blackface ewes which the previous year had borne BD affected lambs after infection with II B pool (Moredun BD virus) were examined serologically. Both groups had neutralizing antibody to NADL strain of bovine virus diarrhoea virus (BVD) and to a cytopathic BD virus isolated from the II B pool. The Mule sheep had higher titres to BVD than to BD and this situation was reversed in the Blackface. In their next pregnancy each group was sub-divided and challenged with the homologous or heterologous strain of virus. Ewes resisted challenge with the homologous strain but 92 and 50 per cent were susceptible to challenge with the heterologous strain. The findings may explain some of the conflicting results encountered in the literature on immunity to BD.