Tau protein is a microtubule-associated protein implicated in the spatial and temporal specification of microtubules and has been found in the neurofibrillary tangles of Alzheimer's disease. Determination of tau protein structure has revealed three 18 amino acid repeated sequences hypothesized to be tubulin binding sites. Using tau cDNA clones from human fetal brain, we employed E. coli expression systems to synthesize tau protein and fragments of tau protein in order to identify the microtubule binding site. A fragment containing the three repeated sequences binds microtubules, while the amino-terminal half of the protein does not bind. Fragments containing two or one repeat are also capable of binding, indicating that the basic tubulin interacting unit is one repeat.