In recent years, we have used a variety of tau immunological markers combined with the dye thiazin red (TR), an accurate marker to differentiate the fibrillar from the nonfibrillar state of both amyloid-beta and tau in Alzheimer's disease (AD). In this study, we used TR as a potential diagnostic marker of AD in frozen-thawed (F-T) brain tissue and imprint cytology. Control experiments included the use of Thioflavin-S staining, fixed tissue, and some double-labeled material with TR and selected tau markers, including AT100, MC1, Alz-50, TG-3, Tau-C3, and S396. Our results indicate that TR retains its strong affinity for both tangles and plaques in unfixed F-T tissue and imprint cytology. This information provides a potential use of TR as an accurate diagnostic tool for the rapid postmortem diagnosis of AD neuropathology. This study shows the advantages of TR on cytology mainly because tools for the fast postmortem diagnosis of AD are practically nonexistent. In addition, we observed Tau-C3 immunoreactivity in extracellular tangles, suggesting that the Tau-C3 epitope is characteristically stable. Moreover, this study demonstrates that chemical fixation is not necessarily required for tau immunoreactivity on histological sections.