Clonal selection of two human prostate carcinoma cell lines (E4 and E9) has resulted in stable expression of approx. 5-fold resistance to the antimicrotubule drug estramustine. Drug-induced antimitotic effects producing IC50 cytotoxicity occur at 12.5 μM (resistant cells) compared to 2.5 μM for the wild type DU145 cells. The E4 and E9 cells are approx. half the size of the wild type, but on a comparative basis showed no difference in the relative quant. protein levels of MAP-4, α or β-tubulin, or β-actin. In contrast to protein expression, Northern blot anal. revealed that in E4 and E9 cells, mRNA levels for MAP-4, β-tubulin and β-actin were elevated 10-fold when compared to the wild type. For α-tubulin, mRNA levels were similar. Although the ratio of soluble vs. polymerized tubulin is known to play a role in autoregulation of β-tubulin mRNA, no significant difference in this ratio was observed between the WT and resistant cells. Immunofluorescent microscopy showed that E4 and E9 cells when compared to the wild type have similar microtubule organization but altered microfilament organizing centers with pronounced actin bundling. Overall, these data are consistent with the principle that the antimicrotubule properties of estramustine can cause an imbalance in the transcripts coding for both major microtubule and microfilament proteins, suggesting that coordinate regulatory mechanisms may be affected.