Taxol, a microtubule stabilizing agent, has been used to study changes in spindle microtubule organization during mitosis. PtK1 cells have been treated with 5 μg/ml taxol for brief periods to determine its effect on spindle architecture. During prophase taxol induces microtubules to aggregate, particularly evident in the region between the nucleus and cell periphery. Taxol induces astral microtubule formation in prometaphase and metaphase cells concomitant with a reduction in spindle length. At anaphase taxol induces an increase in length in astral microtubules and reduces microtubule length in the interzone. Taxol-treated telophase cells show a reduction in the rate of furrowing and astral microtubules lack a discrete focus and are arranged more diffusely on the surface of the nuclear envelope. In summary, taxol treatment of cells prior to anaphase produces an increase in astral microtubules, a reduction in kinetochore microtubules and a decrease in spindle length. Brief taxol treatments during anaphase through early G1 promotes stabilization of microtubules, an increase in the length of astral microtubules and a delayed rate of cytokinesis.