During anaphase B spindle elongation, interzonal microtubules lengthen to accomplish pole-pole separation, while at the same time remaining highly dynamic [Shelden and Wadsworth, J. Cell Sci. 97:273-281, 1990]. To further examine the role of microtubule polymerization and dynamics during spindle elongation, cells have been treated with taxol, which induces microtubule polymerization and stabilizes microtubules. Taxol was added to PtK1 cells 3 minutes after initial chromatid separation, so that the effect on anaphase B could be observed with minimal disruption to anaphase A movement. In 20 microM taxol, the rate and extent of pole-pole separation, measured from time-lapse video records, are reduced to 4% and 9.5% of controls, respectively. The organization of microtubules in taxol treated cells was examined using tubulin immunofluorescence and confocal fluorescence microscopy. Taxol induces a dramatic reorganization of interzonal microtubules resulting in a narrow gap, which is nearly completely lacking in MTs, across the center of the interzone. Furthermore, microtubules in taxol treated cells are resistant to nocodazole induced microtubule disassembly. Our results reveal that taxol rapidly inhibits anaphase B spindle elongation; inhibition is accompanied by a depletion of interdigitated interzonal microtubules and a reduction in microtubule dynamic behavior.