Ten monoclonal antibodies have been raised against lysates of cells infected with cytopathic border disease virus (BDV). These antibodies all recognize non-cytopathic BDV and react with a number of different strains of bovine viral diarrhoea virus (BVDV). Studies with radiolabelled cell lysates show that all the antibodies precipitate two polypeptides of apparent Mr 80,000 and 130,000 from cells infected with cytopathic virus and a single polypeptide of apparent Mr 120,000 from cells infected with non-cytopathic virus. Two of the monoclonal antibodies react on immunoblots and show the same pattern of reactivity indicating that these three polypeptides are antigenically related.