Bovine testicle (BT) cells infected with non-cytopathogenic (NCP) bovine viral diarrhoea virus (BVDV) developed cytopathogenic effect (CPE) after superinfection with 7 Orbiviruses, whereas no CPE was induced by them in the absence of NCP BVDV infection. The CPE was accompanied by the enhanced replication of Orbiviruses. Seven of 10 strains of NCP BVDV induced the enhanced replication of Ibaraki virus, a member of Orbivirus. These 7 strains of NCP BVDV were END phenomenon positive. In contrast, the absence of CPE and the suppression of growth of Ibaraki virus were seen in BT cells infected with the other 3 strains which were END phenomenon negative. The END phenomenon negative viruses were different markedly from the END phenomenon positive viruses with respect to interactions with Orbivirus. The mechanism of the enhanced replication of Orbivirus seems to be explained with the suppression by the END phenomenon positive NCP BVDV to the interferon production of Orbivirus in BT cells.