Effects of estrogens on the cytoplasmic microtubule network were examined by the indirect immunofluorescence method using anti-β-tubulin antibody. Estradiol, a naturally occurring estrogen, decreased the amount of cytoplasmic microtubule fibers during interphase in the human breast cancer cell lines MCF-7 and MDA-MB-231. Since MDA-MB-231 is an estrogen receptor-negative cell line, estradiol-induced microtubule disruption seems to be independent of estradiol binding to receptors. The effective concentration of estradiol required for induction of microtubule disruption in 50% of the cells (EC50) was 81 μM for MCF-7 cells and 82 μM for MDA-MB-231 cells. A synthetic estrogen, diethylstilbestrol, also induced a decrease in microtubule fibers, with an EC50 value of 48 μM for MCF-7 cells and 50 μM for MDA-MB-231 cells. When estrogentreated and microtubule-depolymerized cells were washed and the medium was replaced with fresh, intracellular microtubule networks reappeared within 3 h. When MCF-7 cells were cultured for 4 days with estradiol (50 μM), cell growth was completely inhibited. However, estrone affected the microtubule network and cell proliferation only slightly. These results suggest that estradiol-induced microtubule disruption is closely related to its inhibitory effect on cell growth.