Inocula derived from two sources of typical border disease (BD) and designated BP-77 and H-77 respectively, were prepared and injected into pregnant ewes of four breeds. The BP-77 inoculum produced higher serum neutralising antibody titres to the cytopathic BD virus than to the NADL strain of bovine virus diarrhoea virus. Only typical BD occurred among the progeny of all four breeds. In contrast H-77 produced higher titres to bovine virus diarrhoea than to BD and typical BD occurred only in the progeny of two of the four breeds. Progeny of the other two breeds appeared normal or had a range of malformations of the central nervous system but none of the characteristic clinical or pathological stigmata of BD. The existence of more than one strain of BD virus is confirmed. Virus strain/host genotype interactions may affect the character of disease in the progeny. The possible implications for diagnosis and control are discussed. It is concluded that the generally accepted criteria for diagnosis of BD are no longer adequate and that the disorder should be redesignated.