Localization of amyloid beta/A4 protein precursor (APP) in Alzheimer's neurofibrillary tangles (NFT) was examined immunohistochemically. Antiserum directed to N-terminal of APP intensely labeled intracellular NFT and some neuropil threads. The NFT, extracted from Alzheimer brains by detergent treatments, were also immunoreactive with this antiserum. Antisera to other parts of APP labeled NFT after the formic acid pretreatment. However, Western blot analysis of NFT demonstrated no immunoreactive bands with APP antiserum. These findings suggest that APP is a minor component of the NFT.