The benzothiazole dye thioflavin T (ThT) is a classic amyloid stain for senile plaques containing βA4 peptide in Alzheimer's disease brain. ThT also binds rapidly and specifically to the anti-parallel β-sheet fibrils formed from synthetic β-amyloid (1-42), but does not bind to monomer or oligomeric intermediates. The fibrillar β-sheet-bound dye species undergoes a characteristic 120 nm red shift of its excitation spectrum that may be selectively excited at 450 nm, resulting in a fluorescence signal at 482 nm. ThT is a useful probe for the aggregated fibrillar state of β-amyloid (1-42) fibrils as the amyloid-specific fluorescence reports only fibrillar species. The binding of ThT does not interfere with the aggregation of this peptide into amyloid fibrils. The putative conformational changes detected by the ThT fluorescence suggest that small pharmacologic ligands can perturb and possibly dissociate Aβ amyloid fibrils.