A 10-year-old Holstein dairy cow was slaughtered because of weight loss and ataxia. In addition to a neoplastic mass in the animal's forehead, there was extensive neoplastic infiltration of the skeleton and liver. Other viscera were spared. Tumours were composed of sheets and interlacing fascicles of poorly differentiated, vimentin-positive cells in a fibrillar matrix. Intracytoplasmic virus-like particles, 80 nm in diameter, with a central electron-dense core were found in many neoplastic cells. This neoplasm had an unusual predilection for bone. The significance of the virus-like particles requires further investigation.