Bluetongue virus (BTV) infection results in disparate clinical syndromes among ruminant species. An in vitro model system of BTV/target cell interaction was developed using umbilical vein endothelial cells (EC)from fetal lambs and calves. These cells had microscopic, ultrastructural, and immunocytochemical features typical of EC. BTV infection in these cells was examined using virus binding assays, plaque assays, a whole-cell enzyme-linked immunosorbent assay, flow cytometry, electron microscopy, and a bioassay for interferon activity. EC from both species supported cytopathic BTV infections. Ovine EC bound more BTV initially and produced more virus over time, whereas bovine EC underwent more rapid lysis subsequent to infection. An ultrastructural comparison of BTV-infected ovine and bovine EC, grown as differentiated capillary-like cords on a laminin-rich matrix or as monolayers, revealed no significant interspecies differences in viral morphogenesis between 1 minute and 24 hours after infection. The intracellular distribution of BTV nonstructural protein 1, which localized to virus inclusion bodies and tubules, was identical for ovine and bovine endothelial cells. Ovine and bovine EC produced a soluble mediator of interferon activity in response to BTV infection; however, ovine EC produced higher levels of interferon activity at lower levels of infection. These findings indicate differences in BTV-EC interaction that may contribute to the pathogenesis of the severe inflammatory disease that is characteristic of clinical bluetongue disease in sheep.