The pathogenesis of BD virus infection of sheep is reviewed briefly. The most serious consequences occur when susceptible pregnant sheep are infected. The virus crosses the placenta readily and can cause foetal death with resorption, mummification or stillbirths. Other lambs survive, however, and are born with varying degrees of tremor and/or hairy fleeces. Many such "hairy-shaker" lambs die shortly after birth but survivors gradually recover. These survivors plus other apparently normal lambs can be persistently infected with virus and excrete it constantly often for the rest of their lives. It is these persistently infected sheep that are the key to the epidemiology of the virus and may be responsible for its introduction into a susceptible flock. In addition, cattle grazed with sheep are potentially an important source of pestiviruses capable of causing BD since in some herds the prevalence of cattle persistently infected with pestivirus is one to three per cent. Results are presented of studies on antigenic relation ships among Scottish isolates of cattle pestiviruses (BVD) and BD viruses, and the conclusion drawn that while a single strain of BVDV may be suitable for use as a cattle vaccine this would not be very efficacious in preventing BD since two antigenically distinguishable strains of BD virus have been identified. An effective vaccine against BD would have to protect sheep against both these strains and ideally a cattle vaccine should also contain both strains.