A disturbance of the microtubule network in neurons containing neurofibrillary tangles (NFT), one of the characteristic neuropathological lesions in Alzheimer's disease, has been advocated as a central physiopathological mechanism leading to neuronal dysfunction in NFT-containing neurons. The accumulation of "paired helical filament-tau," the main proteineous component of NFT, in affected neurons has been proposed to induce a decrease of microtubule stability in these cells. To further explore the hypothesis of a decrease in microtubule stability in Alzheimer's disease, we have investigated in this study the relative content in stable microtubules in neurons with different amount of NFT. We used a double immunocytochemical labeling technique with antibodies to tau (as a marker of NFT) and to acetylated alpha-tubulin (as a marker of stable microtubules) and rated on a semi-quantitative scale the tau and acetylated alpha-tubulin-immunoreactivities in the same neurons. We observed a strong reduction in acetylated alpha-tubulin immunoreactivity in most NFT-bearing neurons; a statistical relationship between tau and acetylated alpha-tubulin immunoreactivity was demonstrated, assuming an inverse relationship between the presence of tau-immunoreactive NFT and tubulin-immunoreactivity in neurons. This reduction was already seen in the neuronal population with a relatively lower tau-immunoreactivity, suggesting that reduction in acetylated alpha-tubulin immunoreactivity, and reduction in microtubule stability, could be an early event in these cells.