Secondary deposition of beta amyloid within extracellular neurofibrillary tangles in Alzheimer-type dementia. The hippocampal areas of 34 autopsy specimen brains from aged demented and nondemented subjects were examined using double staining of Bodian and beta protein. In 18 cases (75.5 +/- 7.4 years old), none of the extracellular neurofibrillary tangles (E-NFTs) were immunoreactive with beta protein. In 16 cases (82.9 +/- 5.4 years old), the minority of E-NFTs were immunoreactive with beta-protein antiserum. These beta-immunoreactive E-NFTs frequently appeared in the areas having senile plaques, while they were not observed in the area lacking beta-immunoreactive senile plaques. The ultrastructure of beta-immunoreactive E-NFTs revealed that they consisted of extracellular amyloid fibrils, extracellularly located bundles of paired helical filaments, astroglial processes and degenerating neurites. These findings suggest that the beta immunoreactivity of E-NFTs comes from secondary deposition of amyloid fibrils. 