Engagement of the T cell receptor induces the activation of several mitogen-activated protein kinase modules, including the extracellular signal-regulated kinase and c-Jun N-terminal kinase (JNK) cascades. Whereas extracellular signal-regulated kinase is activated by T cell receptor/CD3 ligation alone, activation of JNK requires co-stimulation by the CD28 receptor. Activation of MEKK-1, which acts as a mitogen-activated protein kinase kinase kinase in the JNK pathway, was also induced by CD3 plus CD28 (CD3/CD28) ligation in Jurkat cells. To study the significance of the JNK cascade in T lymphocytes, we established stable Jurkat cell lines that inducibly express dominant active (DA) or dominant negative (DN) MEKK-1. Whereas expression of DA-MEKK-1 resulted in the constitutive activation of JNK along with the transcriptional activation of the minimal interleukin-2 (IL-2) promoter, DN-MEKK-1 inhibited JNK responsiveness during CD3/CD28 co-stimulation. In addition to inhibiting CD3/CD28-induced IL-2 mRNA expression, DN-MEKK-1 abrogated the transcriptional activation of the IL-2 promoter and the distal nuclear factor of activated T cells (NFAT)-activating protein 1 (AP-1) response element in that promoter. A c-Jun mutant lacking activation sites for JNK also interfered with the activation of the distal NFAT/AP-1 complex, suggesting that the JNK pathway functions by controlling AP-1 response elements in the IL-2 promoter. Using inducible stable expression of DA- and DN-Ras in Jurkat cells, we found that Ras regulates JNK activation in these cells. Our results suggest that the dual ligation of CD3 and CD28 in T cells triggers a cascade of events that involve Ras, the JNK cascade, and one or more AP-1 response elements in the IL-2 promoter.