Taxol, a potent antitumor drug derived from the plant *Taxus brevifolia*, inhibits the replication of human HeLa and mouse fibroblast cells by blocking them in the G2 and M phases of the cell cycle. It stabilizes cytoplasmic microtubules, which are essential components of the cell's cytoskeleton. Taxol-treated cells display bundles of microtubules that appear to radiate from a common site, in addition to their normal microtubules. These microtubules are resistant to depolymerization by cold or 4 mM CaCl₂, indicating that taxol stabilizes microtubules. Taxol also inhibits the migration behavior of fibroblast cells, but these cells remain capable of producing mobile surface projections such as lamellipodia and filopodia. Taxol enhances the rate, extent, and nucleation phase of microtubule polymerization in vitro and stabilizes microtubules. Microtubules assembled in the presence of taxol are resistant to depolymerization by cold or 4 mM CaCl₂. The optimal effects of taxol on microtubule polymerization and stabilization are observed near stoichiometric equivalence with tubulin dimers. Taxol inhibits the migration of fibroblast cells, but these cells are still able to produce lamellipodia and filopodia. This suggests that taxol inhibits cell migration without affecting cell motility. The results indicate that taxol may act by preventing the depolymerization of microtubules, which is essential for cell migration. The inhibition of HeLa and BALB/c fibroblast cell replication and the inability of 3T3 fibroblasts to migrate in the presence of low concentrations of taxol may be due to the cell's inability to depolymerize its microtubule cytoskeleton. Taxol is a potent inhibitor of microtubule polymerization and stabilizer of microtubules. It is a useful tool for studying the regulation of microtubule assembly and cellular functions such as cell migration that may be mediated by microtubules. The drug could also provide a method for the isolation of intact cytoplasmic and spindle microtubules.Taxol, a potent antitumor drug derived from the plant *Taxus brevifolia*, inhibits the replication of human HeLa and mouse fibroblast cells by blocking them in the G2 and M phases of the cell cycle. It stabilizes cytoplasmic microtubules, which are essential components of the cell's cytoskeleton. Taxol-treated cells display bundles of microtubules that appear to radiate from a common site, in addition to their normal microtubules. These microtubules are resistant to depolymerization by cold or 4 mM CaCl₂, indicating that taxol stabilizes microtubules. Taxol also inhibits the migration behavior of fibroblast cells, but these cells remain capable of producing mobile surface projections such as lamellipodia and filopodia. Taxol enhances the rate, extent, and nucleation phase of microtubule polymerization in vitro and stabilizes microtubules. Microtubules assembled in the presence of taxol are resistant to depolymerization by cold or 4 mM CaCl₂. The optimal effects of taxol on microtubule polymerization and stabilization are observed near stoichiometric equivalence with tubulin dimers. Taxol inhibits the migration of fibroblast cells, but these cells are still able to produce lamellipodia and filopodia. This suggests that taxol inhibits cell migration without affecting cell motility. The results indicate that taxol may act by preventing the depolymerization of microtubules, which is essential for cell migration. The inhibition of HeLa and BALB/c fibroblast cell replication and the inability of 3T3 fibroblasts to migrate in the presence of low concentrations of taxol may be due to the cell's inability to depolymerize its microtubule cytoskeleton. Taxol is a potent inhibitor of microtubule polymerization and stabilizer of microtubules. It is a useful tool for studying the regulation of microtubule assembly and cellular functions such as cell migration that may be mediated by microtubules. The drug could also provide a method for the isolation of intact cytoplasmic and spindle microtubules.