Ceratamine A and ceratamine B are heterocyclic alkaloids recently identified in a screen for compounds that arrest cells in mitosis. Treatment of breast carcinoma MCF-7 cells causes a concentration-dependent block of cell cycle progression exclusively at mitosis. In vitro studies with purified tubulin indicate that the ceratamines directly stimulate microtubule polymerization in the absence of microtubule-associated proteins. Cells treated with ceratamines show a dense perinuclear microtubule network in interphase and multiple pillar-like tubulin structures in mitotic cells. The ceratamines do not compete with paclitaxel for binding to microtubules in vitro. Unlike other microtubule-stabilizing agents, the ceratamines have simple structures with no chiral centers, making them attractive drug leads.