Paclitaxel (PTX), the diterpene alkaloid, is a potent anti-cancer drug and is routinely used for the treatment of breast and ovarian cancers. The cellular targets of PTX are microtubules, which are composed of alpha- and beta-tubulin. Development of PTX resistance in patients has been a major problem associated with cancer chemotherapy. In an effort to get insight into this phenomenon of drug resistance, a PTX-resistant cell line from MCF-7 breast cancer cells has been generated. Western analysis of the cell extracts revealed that the resistant cells contain 2-fold higher amount of tyrosinated alpha-tubulin than those of the wild-type MCF-7 cells. Similar analyses of beta-tubulin with the isotype-specific monoclonal antibodies demonstrated that the PTX-resistant cells contain 2.5-fold higher amounts of beta(III) and 1.5-fold higher amount of beta(IV)-tubulin, while no difference was observed in the level of beta(I) isotype. These results demonstrate for the first time that PTX resistance is associated with an increase in the level of tyrosinated alpha-tubulin.