Genistein, a natural isoflavone phytoestrogen present in soybeans, has been extensively studied as a chemopreventive or therapeutic agent in several types of cancer. The traditional Asian diet is rich in soy products may explain in part why the incidence of breast cancer in Asian women is relatively low. To improve therapeutic benefits, we investigated the combination of genistein with chemotherapeutic agents in phenotypically dissimilar human breast cancer cells, MCF-7 and MDA-MB-231, in which estrogen receptor expression is positive and negative, respectively. In the present study, genistein significantly decreased cell apoptosis induced by tubulin-binding agents, paclitaxel and vincristine. FACScan analysis revealed that genistein also diminished the accumulation of the G2/M phase in the cell cycle caused by tubulin-binding agents. In situ staining of microtubules revealed that genistein could decrease paclitaxel-induced tubulin polymerization. However, in vivo tubulin polymerization assay revealed that simultaneous treatment of genistein did not change the tubulin/microtubule dynamic. Genistein reduced Bcl-2 phosphorylation triggered by paclitaxel and vincristine without changing Bax protein expression. p53 and p21 expression, monitored by Western blotting, was not altered by genistein. However, the expression of cyclin B1 and CDC2 kinase was markedly decreased in combination with genistein. In conclusion, genistein inversely affected tubulin-binding agent-induced apoptosis via down-regulation of cyclin B1/CDC2 kinase expression resulting in reduced Bcl-2 phosphorylation.