Newer therapeutic strategies for the treatment of multiple myeloma have focused on antagonizing the growth-promoting functions of interleukin 6 (IL-6). In this study, we examined the antitumor effects of two mechanistically different microtubule poisons, Taxol and vinblastine, in U266 human myeloma cells and determined whether IL-6 altered these effects. Taxol and vinblastine led to a dose-dependent inhibition of [3H]thymidine incorporation and altered the DNA distribution pattern of U266 cells. Both drugs led to an increase in the proportion of cells in the sub-G1 fraction (<2N DNA). However, at the IC50 concentration, vinblastine, but not Taxol, increased the percentage of cells in the G2-M phase of the cell cycle. In the presence of IL-6, the DNA distribution pattern induced by Taxol or vinblastine was altered. Whereas IL-6 augmented the sub-G1 fraction and G2-M phase for Taxol-treated cells, only the G2-M phase was increased for vinblastine-treated cells. Furthermore, IL-6 enhanced the cytotoxicity of both drugs, which became evident only during recovery in cytokine-free and drug-free medium. However, the cytotoxicity of Taxol was augmented to a significantly greater extent than that of vinblastine (P < 0.001). Immunostaining with antibodies to alpha-tubulin and mitogen-activated protein kinase revealed colocalization of these two proteins within microtubule asters. In the presence of IL-6, the number of cells containing microtubule asters increased for Taxol treatment, but not for vinblastine treatment. These data indicate that IL-6 leads to differential modulation of the cytotoxicity of Taxol and vinblastine in U266 cells. Whereas recruitment of cells in the S phase of the cell cycle represents a major mechanism by which IL-6 potentiates the cytotoxicity of vinblastine, augmentation of the cytotoxicity of Taxol involves additional mechanisms. Furthermore, our data suggest that the microtubule-associated form of mitogen-activated protein kinase may play a role in IL-6-mediated enhancement of the cytotoxicity of Taxol. The clinical implications of these findings are discussed.