Maytansine and its analogues (maytansinoids) are potent microtubule-targeted compounds that inhibit proliferation of cells at mitosis. Antibody-maytansinoid conjugates consisting of maytansinoids (DM1 and DM4) attached to tumor-specific antibodies have shown promising clinical results. To determine the mechanism by which the antibody-DM1 conjugates inhibit cell proliferation, we examined the effects of the cleavable anti-EpCAM-SPP-DM1 and uncleavable anti-EpCAM-SMCC-DM1 conjugates on MCF7 human breast tumor cells. We also examined the effects of the free maytansinoids, maytansine and S-methyl DM1 (a version of DM1 that is stable in cell culture medium), for comparison. Both the conjugates and free maytansinoids potently inhibited MCF7 cell proliferation at nanomolar and subnanomolar concentrations, respectively, by arresting the cells in mitotic prometaphase/metaphase. Arrest occurred in concert with the internalization and intracellular processing of both conjugates under conditions that induced abnormal spindle organization and suppressed microtubule dynamic instability. Microtubule depolymerization occurred only at significantly higher drug concentrations. The results indicate that free maytansinoids, antibody-maytansinoid conjugates, and their metabolites exert their potent antimitotic effects through a common mechanism involving suppression of microtubule dynamic instability.