Paclitaxel (PTX) and vinblastine (VBL) represent 2 classes of drugs that target tubulin but have separate binding properties and opposing mechanisms of action. To evaluate the potential use of these agents together in a chemotherapeutic regimen, we investigated their effects on the dynamics of tubulin polymerization and cellular cytotoxicity, when administered singly or in combination. In human epidermoid carcinoma KB cells and MCF-7 breast carcinoma cells, we observed a time- and dose-dependent effect on cytoskeletal dynamics for both PTX and VBL. Tubulin polymerization induced by PTX was stable for more than 24 hr. When PTX treatment was followed by VBL, a time- and dose-dependent reversal of tubulin polymerization was observed. In contrast, rapid tubulin polymerization occurred when VBL was followed by PTX. When both agents were added simultaneously, a diminution of PTX-induced tubulin polymerization was observed with increasing doses of VBL; a maximum reduction was achieved when equal concentrations were used. Examination of the tubulin pattern by immunofluorescence in MCF-7 breast cancer cells confirmed and extended our findings. Bundle formation followed treatment with PTX. Addition of increasing concentrations of VBL prevented bundling; however, the normal cytoskeletal architecture was not restored. Cytotoxicity studies carried out using the median dose effect principles and the combination index analysis showed synergism when VBL and PTX were administered sequentially and antagonism for simultaneous administration. Our results demonstrate changes in tubulin dynamics following drug treatment and provide a rationale for combined PTX/VBL therapy after careful evaluation of the schedule of administration.