Polyglucosan bodies (PGBs) in the brain of a 12-year-old Holstein cow exhibiting no signs of neurological abormality were examined by light and electron microscopy and immunohistochemistry. PGBs were disseminated throughout the brain, especially in the pallidum, thalamus and cerebellum. Cow PGBs were found in the neuronal perikaryon and in the neuropil. These were round, slightly to severely basophilic, and were strongly positive for the periodic acid-Schiff reaction. The cow PGBs were immunoreactive for monoclonal antibodies raised against human polyglucosan. Electron microscopic analysis revealed that they were composed of branching filaments, glycogen granules and electron-dense material. These findings indicate that cow PGBs closely resemble human or canine PGBs in Lafora's disease and PGBs found in aged animals that have been reported previously.