Neuronal processes contain high concentrations of two related microtubule-associated proteins, MAP2 and tau. When MAP2 is expressed in non-neuronal cells the microtubules appear to be stiffer than those in control cells that do not express MAP2. A stiffening effect of MAP2 is further suggested by recent experiments with microtubules reassembled in vitro and by the fact that, under appropriate circumstances, MAP2-expressing cells can be induced to form processes that are long and cylindrical. Both MAP2 and tau contain homologous microtubule-binding domains, consisting of three or four repeats of an 18 amino acid sequence, which we believe are responsible for the stiffening effect. Our hypothesis is that each repeat binds to a neighbouring tubulin subunit in the wall of the microtubule, tethering them together and reducing their freedom of movement relative to one another. Based on these considerations, we suggest that MAP2 and tau may contribute to the support of neuronal processes by making the microtubules they contain longer, more stable and stiffer than those in non-neuronal cells.