Assessment of specific apoptosis and survival pathways implicated in anticancer drug action is important for understanding drug mechanisms and modes of resistance in order to improve the benefits of chemotherapy. In order to better examine the role of mitogen-activated protein kinases, including JNK and ERK, as well as the tumor suppressor p53, in the response of tumor cells to chemotherapy, we compared the effects on these pathways of three structurally and functionally distinct antitumor agents. Drug concentrations equal to 50 times the concentration required to reduce cell proliferation by 50% were used. Vinblastine, doxorubicin, or etoposide (VP-16) induced apoptotic cell death in KB-3 carcinoma cells, with similar kinetic profiles of PARP cleavage, caspase 3 activation, and mitochondrial cytochrome c release. All three drugs strongly activated JNK, but only vinblastine induced c-Jun phosphorylation and AP-1 activation. Inhibition of JNK by SP600125 protected cells from drug-induced cytotoxicity. Vinblastine caused inactivation of ERK whereas ERK was unaffected in cells exposed to doxorubicin or VP-16. Inhibition of ERK signaling by the MEK inhibitor, U0126, potentiated the cytotoxic effects of vinblastine and doxorubicin, but not that of VP-16. Vinblastine induced p53 downregulation, and chemical inhibition of p53 potentiated vinblastine-induced cell death, suggesting a protective effect of p53. In contrast, doxorubicin and VP-16 induced p53, and inhibition of p53 decreased drug-induced cell death, suggesting a pro-apoptotic role for p53. These results highlight the differential roles played by several key signal transduction pathways in the mechanisms of action of key antitumor agents, and suggest ways to specifically potentiate their effects in a context-dependent manner. In addition, the novel finding that JNK activation can occur without c-Jun phosphorylation or AP-1 activation has important implications for our understanding of JNK function.