The peroxidase-antiperoxidase technique was used to determine the cellular localization of Border Disease (BD) virus in cryostat sections of fetal and newborn lamb spinal cord following experimental infection by maternal inoculation in early gestation. Viraemic fetuses and lambs with hypomyelinogenesis showed BD viral antigen in neurons, glia, ependymal cells, vascular endothelial cells and fibrocytes within the dura mater. Double immunolabelling demonstrated co-expression of BD viral antigen and glial fibrillary acidic protein (GFAP) or myelin basic protein (MBP) in both fetal and newborn lamb glia. In fetal lambs there was a pia-associated population of glia in which viral antigen was also co-expressed with GFAP or MBP. The results suggest that BD virus infects myelinating oligodendroglia, astroglia and probably also transitional cells and pluripotential glioblasts. The relationship between infection of specific cell types and hypomyelinogenesis was not resolved but infection of transitional cells and oligodendroglia may affect oligodendroglial function and permit morphologically inapparent perturbations leading to hypomyelinogenesis. A single nonviraemic lamb with a precolostral antibody titre to BD virus and cystic cerebral cavities but no hypomyelinogenesis showed BD viral antigen confined to glia of the spinal cord white matter. This suggests that oligodendroglia may require to be infected before a critical period in their development or factors additional to oligodendroglia infection are necessary for hypomyelinogenesis.