Infectious bovine rhinotracheitis virus (IBRV) established long-term persistent infection in intracerebrally inoculated athymic nude mice. After intraperitoneal injection into outbred mice, virus was isolated for only 3 days from spleens and livers. In vitro inoculation of outbred mouse spleen fragments with IBRV resulted in persistent infection and subsequent transformation of spleen macrophages. The IBRV-specific membrane and intracellular antigens were detected by indirect immunofluorescence techniques in transformed cells in early in vitro passages. The presence of IBRV genetic formation was confirmed by in situ hybridization. The IBRV-transformed mouse macrophages induced fibrosarcomas and cystic tumors in athymic nude mice. Infective virus could not be rescued from transformed cells by cocultivation with rabbit kidney cells, treatment with iododeoxyuridine, or ultraviolet irradiation.