The combined activity of a stabilizing and a depolymerizing agent was studied on microtubule formation in vitro and on cellular parameters related to the cytoskeleton. New compounds in each class of microtubular drugs, docetaxel, as stabilizing agent, and CI 980, as colchicine analog, currently in clinical trials, were tested at high and low concentrations. Simultaneous combination of docetaxel and CI 980, both in vitro and in cell lines, induced microtubule structures resulting from the association of the effects of the two drugs: short and numerous microtubules in vitro, abnormal asters or short bundles in cells depolymerized in their periphery. Moreover, combining the two drugs at low concentrations inducing neither modification of the microtubular network nor variation in cell polymerized tubulin content showed synergistic effects in mitotic cell block and, at lower concentrations, in inhibition of proliferation in the KB 3-1 cell line. Similar qualitative results were obtained with paclitaxel and colcemid, used in the place of docetaxel and CI 980, respectively. At low doses, the taxoid/colchine analog combination seems to induce some degree of mitotic inhibition, resulting from a subtle effect on the inhibition of microtubule dynamics.