The paired helical filament, the principal constituent of the neurofibrillary tangles characteristic of Alzheimer disease, is shown to consist of two structurally distinct parts. An external fuzzy region can be removed by Pronase treatment to leave a Pronase-resistant morphologically recognizable core. Scanning transmission electron microscopy gives an estimate for the mass per unit length as 79 kDa.nm-1 before Pronase treatment and 65 kDa.nm-1 after treatment. The fuzzy region carries all the epitopes recognized by two different antisera against microtubule-associated protein tau. By contrast, a monoclonal antibody (mAb) we have raised to paired helical filament cores (mAb 423) decorates Pronase-treated filaments much more strongly than it does untreated ones. We have shown in previous papers that the epitope recognized by mAb 423 is carried by a central 9.5-kDa fragment of tau protein, which therefore forms part of the Pronase-resistant core structure. The remainder of the tau protein incorporated into the filaments must contribute part, if not all, of the fuzzy region. The mass per unit length measurements imply that the three-domain structural subunit of the core that we visualized previously by image reconstruction has a molecular mass of approximately equal to 100 kDa.