Alzheimer's disease is typified by the characteristic histopathological lesions of neurofibrillar plaques and tangles. The latter are composed of paired helical filaments (PHFs), the major components of which are modified forms of the microtubule-associated protein tau. The exact nature of these modifications remains unknown, although the presence of hyperphosphorylated tau in PHFs argues strongly that phosphorylation is one of the modifications that result in the polymerization of tau into PHFs. However, hyperphosphorylation alone is insufficient to explain the formation of PHFs. In an attempt to characterize other post-translational modifications of PHF-tau, we have analyzed its glycation. A fraction of PHF-tau seems to be glycated in vivo, whereas soluble tau from either Alzheimer's disease or non-demented human brain is not glycated at all. Purified tau from bovine brain can be efficiently glycated in vitro. Tau glycation is accompanied by a decrease in the tau binding to tubulin. These results support the view that glycation may be one of the modifications hampering the binding of tau to tubulin in Alzheimer's disease, thus facilitating tau aggregation into PHFs.