A line of Madin Darby canine kidney (MDCK) cells persistently infected with Borna disease virus was examined by electron microscopy. Thin sections revealed the presence of intracytoplasmic virus-like particles ranging from 50-100 nm in diameter. Nuclei of the infected cells exhibited accumulation of electron-dense granular structures 15-18 nm in diameter. The intracytoplasmic particles were roughly spherical with a limiting membrane, suggesting the presence of a lipid-containing envelope. The internal structure consisted of strand-like material which in some cases was condensed underneath the envelope. The possible relationship of these particles to Borna disease virions is discussed.