The search for microtubule-associated proteins (MAPs) in plants is relatively recent. In particular, the "classical MAPs," which stimulate the polymerization and stabilization of microtubules, have only been examined in heterogeneous fractions. As a first step in dissecting the role of individual MAPs, we have chromatographically purified a single 60-kDa protein from a carrot MAP fraction and analyzed its effects on tubulin assembly. MAP60 promoted the formation of long, morphologically regular brain microtubules in vitro, an effect inhibited by preincubation of the MAP with affinity-purified antibodies against this protein. MAP60 also increased the stability of microtubules to dilution and significantly enhanced cold stability to the normally cold-sensitive neurotubules. These in vitro properties are consistent with a role for MAP60 in regulating the turnover/assembly of dynamic plant microtubules in vivo.