Enriched fractions of "tangles" obtained from cases of Alzheimer's disease have been observed by negative staining and platinum: iridium shadowing, and also after treatment with sodium dodecyl sulfate and urea. Abnormal filaments with a 80-nm periodic constriction exhibited a substructure consisting of six protofilaments. They also showed a periodic right-handed twist. Some fibrils with a 160-nm periodic constriction were made up of four protofilaments. These features are discussed in view of known fibrous protein models, especially neurofilaments.