Three groups of five and one group of four oestrus-synchronised sheep were inoculated with Border disease (BD) virus at 52 +/- 2 days after their first service. Transmission of virus to offspring as demonstrated by virus isolation, detection of viral antigen and, or antibody response occurred in 12 of 19 sheep and probably in four others which aborted or produced stillborn lambs. Both apparently normal and clinically affected animals excreted virus in saliva, urine and faeces, and excretion and contact transmission to sheep and pigs persisted for up to two and a half years. Most of the tissues of infected sheep contained virus titres between 10(3.5) and 10(5.5) TCID50 per g. The immune response in the lambs varied, in some it began before birth, in others a transient or low level response was observed in the first or second year, while others remained serologically negative for two and a half years.