The experiments were performed with embryonal chicken fibroblasts, HeLa cells and H. Ep. 2 (human larynx carcinoma) in hanging drop cultures. They confirm the results obtained with tumor bearing mice. Withaferin A is acting as a mitotic poison and arrests the tumor cells at metaphase. Its effect on cells may be compared to that of colchicine, alkaloids of vinca rosea and podophyllin. The results obtained with chicken fibroblasts and HeLa cells indicate that Withaferin A has a stronger action on tumor cells than on normal cells. A synergistic action between Withaferin A and a derivative of colchicine was also demonstrated.