The interaction of the antimitotic agent estramustine with bovine microtubule proteins and purified tubulin was investigated. Direct photoaffinity labeling of microtubule protein with [14C]estramustine resulted in the labeling of both alpha- and beta-tubulin, and this was inhibited with unlabeled estramustine in a dose-dependent manner. [14C]Estramustine was incorporated into both the soluble and polymerized forms of tubulin. The affinity constant for estramustine binding to tubulin was determined by equilibrium dialysis to be 23 +/- 5 mM. Estramustine did not affect [3H]vinblastine binding, and vinblastine had no effect on direct labeling with [14C]estramustine. Both rhizoxin and paclitaxel decreased the covalent labeling of tubulin with [14C]estramustine in a dose-dependent fashion and were noncompetitive inhibitors of the binding of estramustine to tubulin. The binding of colchicine to tubulin was not inhibited by estramustine as detected by fluorescence and DEAE filter assays. The estramustine binding site on tubulin is therefore distinct from that of colchicine and vinblastine and may at least partially overlap with the binding site for paclitaxel. In both bovine brain microtubules and cytoskeletal proteins from human prostatic carcinoma cells, the incorporation of [14C]estramustine into the beta III isotype of tubulin was found to occur with a reduced efficiency compared to that of the other beta-tubulin isotypes and alpha-tubulin. Since this isotype is overexpressed in estramustine resistant human prostate carcinoma cells, these results indicate that beta III-tubulin may play a role in the response to the effects of estramustine.