While phosphorylation of high-molecular-weight microtubule-associated proteins (MAPs) alters the assembly properties of microtubules in vitro, virtually nothing is known about the influence of MAP phosphorylation on the time-scale of microtubule polymer length redistribution. The latter has been used as an index of microtubule assembly/disassembly turnover as predicted by the dynamic instability model (Mitchison, T.M. and Kirschner, M.W. (1984) Nature 312, 237-242). We have now determined that under conditions leading to the incorporation of 8-10 mol phosphoryl groups per mol MAP-2 (and about 0.2 mol phosphoryl groups per mol MAP-1 and tau), we can reproducibly observe significant acceleration in the polymer length redistribution process in a manner consistent with greater microtubule dynamic instability. We have also found that MAP phosphorylation resulted in more extensive release of MAPs from microtubules as a function of increasing salt concentration. These results are consistent with a weakening of MAP-microtubule interactions upon phosphorylation.