Buffy coat of three members from a family with Alzheimer's disease was inoculated into hamster brains. Eighteen months after the inoculation, all experimental animals were sacrificed for the neuropathological study. Hematoxylin-eosin staining showed no gross vacuolar degeneration, or neuronal loss in the cortex. The spongiform degeneration was minimum. Immunostaining with antibodies against neurofilament 200 kDa subunit protein revealed massive immuno-positive intracytoplasmic inclusion bodies within neurons of the brainstem nuclei. By electron microscopy, the intracytoplasmic inclusion body was shown to be composed of proliferated 10 nm neurofilaments. The intra-cytoplasmic neurofilament proliferation was observed with the hamsters inoculated with the buffy coat from Alzheimer's disease patients as well as an apparently normal member of the family.