Glycated residues of tau protein from paired helical filaments isolated from the brains of Alzheimer's disease patients were localized by doing a proteolytic cleavage of the protein, fractionation of the resulting peptides, and identification of those peptides using specific antibodies. The most suitable residues for glycation, lysines, present at the tubulin-binding motif of tau protein, seem to be preferentially modified compared with those lysines present at other regions. Among these modified lysines, those located in the sequence comprising residues 318-336 (in the largest human tau isoform) were found to be glycated, as determined by the reaction with an antibody that recognizes a glycated peptide containing this sequence. Because those lysines are present in a tubulin binding motif of tau protein, its modification could result in a decrease in the interaction of tau with tubulin.