Tau protein is the major component of the intraneuronal filamentous inclusions that constitute defining neuropathological characteristics of Alzheimer's disease and other tauopathies. The discovery of tau gene mutations in familial forms of frontotemporal dementia has established that dysfunction of the tau protein is sufficient to cause neurodegeneration and dementia. Here we have tested 42 compounds belonging to nine different chemical classes for their ability to inhibit heparin-induced assembly of tau into filaments in vitro. Several phenothiazines (methylene blue, azure A, azure B, and quinacrine mustard), polyphenols (myricetin, epicatechin 5-gallate, gossypetin, and 2,3,4,2',4'-pentahydroxybenzophenone), and the porphyrin ferric dehydroporphyrin IX inhibited tau filament formation with IC(50) values in the low micromolar range as assessed by thioflavin S fluorescence, electron microscopy, and Sarkosyl insolubility. Disassembly of tau filaments was observed in the presence of the porphyrin phthalocyanine. Compounds that inhibited tau filament assembly were also found to inhibit the formation of Abeta fibrils. Biochemical analysis revealed the formation of soluble oligomeric tau in the presence of the inhibitory compounds, suggesting that this may be the mechanism by which tau filament formation is inhibited. The compounds investigated did not affect the ability of tau to interact with microtubules. Identification of small molecule inhibitors of heparin-induced assembly of tau will form a starting point for the development of mechanism-based therapies for the tauopathies.