Microtubule inhibitors can be classified into two categories: 1) those which inhibit the polymerization-dependent GTPase activity of phosphocellulose-purified tubulin, but induce a significant polymerization-independent GTPase activity (e.g. colchicine, griseofulvine, daunorubicine); 2) those which inhibit the GTPase activity associated with tubulin polymerization and that induced by inhibitors of the first class (e.g. the vincaalkaloids and podophyllotoxin). The colchicine-stimulated GTPase activity of tubulin appears to be due to the tubulin.colchicine complex. This suggests that colchicine inhibits tubulin assembly by binding to a tubulin-tubulin interaction site required for the polymerization-dependent GTPase activity and induces by itself a tubulin conformational change that leads to polymerization-independent GTPase activity. Stoichiometry of inhibition by vinblastine of the colchicine-stimulated GTPase activity is 1:2. On the other hand, the inhibition by vinblastine of the tubulin self-assembly and of the polymerization-dependent GTPase activity is strongly substoichiometric at the beginning of the polymerization reaction, 1 vinblastine molecule inhibiting the ability of 10 tubulin dimers to polymerize and to hydrolyze the GTP. However, at the polymerization plateau, the inhibition effect by vinblastine appears to be lower, suggesting a selective action of vinblastine on the early stages of the polymerization reaction.