Amyloid B (AB) immunoreactivity has been demonstrated in all extracellular neurofibrillary tangles (E-NFT) and most intraneuronal neurofibrillary tangles (I-NF. We undertook this immunocytochemical study to understand the relationship between AB immunoreactivity localized in NFT and B-protein precursor (BPP). We found epitopes of amino-, mid-, and carboxyl-terminal domains of BPP in I-NFT and the majority of ENFT. NFT retained (BPP after ionic detergent extraction, demonstrating that BPP is an integral component of NFT. Finding BPP in regions of AB immunoreactivity raises the possibility that BPP or its fragments associate with amyloid, and that the stability of AB is responsible for its dominance in amyloid deposits.