Taxol is a clinically active anticancer drug, which exerts its cytotoxicity by the unique mechanism of polymerizing tubulin monomers into microtubules and stabilizing microtubules. Our studies with ovarian (hamster CHO and human A2780) cells showed that taxol is a phase-specific agent that is much more cytotoxic to mitotic cells than interphase cells. First, the dose-survival pattern of taxol resembled that of other phase-specific agents, in which cell-kill reached a plateau at a certain concentration. This suggests that the asynchronous cell population consists of a taxol-sensitive (presumably mitotic) fraction and a taxol-resistant fraction. Second, the cells were more responsive to increased exposure time than to increased dose above the plateau concentration. Third, in both asynchronous and synchronous cultures taxol was much more cytotoxic to mitotic than interphase (G1, S and G2) cells. Fourth, the taxol concentration needed to kill cells corresponded to the dose needed to block cells in mitosis. Although taxol blocked cells in mitosis, the mitotic block was of short duration. Cells escaped the mitotic block, without cytokinesis, and entered the next round of DNA synthesis to form multinucleated polyploid cells. Taxol was 15- to 25-fold more toxic to A2780 (human ovarian carcinoma) cells compared to CHO cells. This difference in sensitivity correlated with a higher intracellular taxol concentration in A2780 as compared to CHO as determined by either an ELISA assay or by [H3]-taxol uptake.