Taxol stabilizes microtubules, prevents tubulin depolymerization, and promotes tubulin bundling and is one of the most effective drugs for the treatment of metastatic breast and ovarian cancer. Although its interaction with tubulin has been well characterized, the mechanism by which taxol induces growth arrest and cytotoxicity is not well understood. Herein, we show that taxol induced dose- and time-dependent accumulation of the cyclin inhibitor p21WAF1 in both p53 wild-type and p53-null cells, although the degree of induction was greater in cells expressing wild-type p53. In MCF7 cells, wild-type p53 protein was also induced after taxol treatment, and this induction was mediated primarily by increased protein stability. Taxol induced both p21WAF1 and wild-type p53 optimally in MCF7 cells after 20-24-h exposure with an EC50(3) of 5 nM. In p53-null PC3M cells, p21WAF1 was similarly induced after 24-h exposure to taxol. Coincident with these biochemical effects, taxol altered the electrophoretic mobility of c-raf-1 and stimulated mitogen activated protein kinase. Previous depletion of c-raf-1 inhibited both the p21WAF1- and p53-inducing properties of taxol, as well as the activation of MAP kinase. These data suggest that induction of p21WAF1 by taxol requires c-raf-1 activity, but that it is not strictly dependent on wild-type p53. Furthermore, the ability of taxol to both induce wild-type p53 in MCF7 cells and activate MAP kinase is also dependent on c-raf-1 expression.