The proinflammatory cytokine tumor necrosis factor alpha (TNF-alpha) regulates immune responses, inflammation, and programmed cell death (apoptosis). TNF-alpha exerts its biological activities by activating multiple signaling pathways, including IkappaB kinase (IKK), c-Jun N-terminal protein kinase (JNK), and caspases. IKK activation inhibits apoptosis through the transcription factor NF-kappaB, whose target genes include those that encode inhibitors of both caspases and JNK. Despite activation of the antiapoptotic IKK/NF-kappaB pathway, TNF-alpha is able to induce apoptosis in cells sensitive to it, such as human breast carcinoma MCF-7 and mouse fibroblast LM cells. The molecular mechanism underlying TNF-alpha-induced apoptosis is incompletely understood. Here we report that in TNF-alpha-sensitive cells activation of the IKK/NF-kappaB pathway fails to block TNF-alpha-induced apoptosis, although its inactivation still promotes TNF-alpha-induced apoptosis. Interestingly, TNF-alpha-induced apoptosis is suppressed by inhibition of the JNK pathway but promoted by its activation. Furthermore, activation of JNK by TNF-alpha was transient in TNF-alpha-insensitive cells but prolonged in sensitive cells. Conversion of JNK activation from prolonged to transient suppressed TNF-alpha-induced apoptosis. Thus, absence of NF-kappaB-mediated inhibition of JNK activation contributes to TNF-alpha-induced apoptosis.