A virus has been demonstrated by inoculating foetal lamb kidney (FLK) cultures with suspensions of pooled brain from newborn unsuckled lambs affected with Border disease (BD) and with suspensions of brain or spleen from individual aborted foetuses whose mothers had been injected with the 29th FLK passage of BD material. Some of its physico-chemical characteristics are reported and are compared with those of the NADL strain of the bovine virus diarrhoea-mucosal disease (BVD-MD) virus. Following the injection of pregnant ewes with supernate from the 12th and 29th passage of BD in FLK cells a high percentage of the ewes aborted, all foetuses examined had histopathological lesions of Border disease, and the only live born lamb was clinically and pathologically a case of Border disease. When the highest buoyant density fraction of virus, purified by limiting dilutions was injected into pregnant ewes the lambs were clinically normal, but histological lesions of BD were detected. The possible explanation for the difference in severity of the Border disease found in different experiments is discussed.