If spirochetes play a role in the formation of senile plaques, neurofibrillary tangles and neuropil threads, we anticipated that they may contain DNA. The 4’6-diamidine-2’-phenylindole dihydrochloride (DAPI) binds selectively to DNA and forms strongly fluorescent DNA-DAPI complexes (Russel et al., 1975). DNA staining is the most popular method for the detection of cell culture Mycoplasma by its DNA content. Using this sensitive DNA stain, not only reference spirochetes may be visualized but senile plaques, neurofibrillary tangles and neuropil threads as well. DNase pretreatment abolishes the DAPI staining of these structures but not their Thioflavin S affinity indicating that they indeed contain DNA (Miklossy et al., 1995).