The paired helical filament (PHF) is the major component of the neurofibrillary deposits that form a defining neuropathological characteristic of Alzheimer's disease. PHFs are composed of microtubule-associated protein tau, in a hyperphosphorylated state. Hyperphosphorylation of tau results in its inability to bind to microtubules and is believed to precede PHF assembly. However, it is unclear whether hyperphosphorylation of tau is either necessary or sufficient for PHF formation. Here we show that non-phosphorylated recombinant tau isoforms with three microtubule-binding repeats form paired helical-like filaments under physiological conditions in vitro, when incubated with sulphated glycosaminoglycans such as heparin or heparan sulphate. Furthermore, heparin prevents tau from binding to microtubules and promotes microtubule disassembly. Finally, we show that heparan sulphate and hyperphosphorylated tau coexist in nerve cells of the Alzheimer's disease brain at the earliest known stages of neurofibrillary pathology. These findings, with previous studies which show that heparin stimulates tau phosphorylation by a number of protein kinases, indicate that sulphated glycosaminoglycans may be a key factor in the formation of the neurofibrillary lesions of Alzheimer's disease.