4'-methoxy-2-styrylchromone, a new synthetic chromone was identified as a selective proliferation inhibitor of human tumor (MCF-7 and NCI-H460) cell lines than to non-tumor cells (MRC-5). The antiproliferative activity of this chromone was also extensive to peripheral human lymphocytes. 4'-Methoxy-2-styrylchromone was found to block tumor cells in the G2/M phase of the cell cycle. The G2/M arrest of NCI-H460 cells was dose- and time-dependent, reaching a maximum after 12-h treatment while MCF-7 cells reached the maximum value of G2/M accumulation only after a 24-h treatment. Chromone-treated cells evidenced a high frequency of cells in prometaphase, indicating progression beyond G2 and arrest early in mitosis. This mitotic arrest was associated with abnormal mitotic spindles characterized by the formation of a monopolar structure, suggesting that the chromone interferes with microtubules. The results of an in vitro tubulin polymerization assay showed that this chromone stabilizes microtubules in a manner similar to paclitaxel.