Filamentous, fibronectin-immunopositive structures, previously described in Alzheimer's disease and control brains were negative for neuronal, glial, and macrophage markers. The present study sought to determine the nature of these entities and to further characterize their morphology, immunoreactivity and distribution between neuropathologies. Ultrastructural analysis shows these formations to be filamentous micro-organisms, which may belong to the actinomycetes. Immunohistochemistry for the cell-stress protein ubiquitin is consistently positive in these organisms. They are also present in Down's syndrome, dementia pugilistica, amyotrophic lateral sclerosis with dementia, and Parkinson's disease. The pattern of tissue distribution implies a pre-mortem invasion of the brain, and, as the micro-organism is present at a four to five-fold higher frequency in Alzheimer's disease, it may act pathogenically in this dementing illness.