We have examined the regulation of the AP-1 transcription complex in the IL-1-responsive murine T cell thymoma cell line EL-4 6.1 C10. Our results demonstrate that AP-1-mediated gene expression in T cells may be regulated by several signaling pathways and factors, including IL-1, protein kinase C, protein kinase A (PKA), and one or more serine/threonine-specific protein phosphatases. The activation of protein kinase C results in an increase in nuclear AP-1 DNA binding activity, as well as enhanced gene expression. IL-1 and agents that elevate intracellular cAMP levels do not, by themselves, induce AP-1 activation, but they synergize with phorbol esters. IL-1 and forskolin may enhance AP-1 function by different mechanisms, because forskolin enhanced gene expression without producing an increase in nuclear AP-1 DNA binding, whereas IL-1 increased AP-1-binding activity and gene expression. These observations, in conjunction with the lack of a demonstrable effect of IL-1 on cAMP production in EL-4 cells, are consistent with the view that IL-1 enhances AP-1 activation by a pathway that does not directly involve cAMP and PKA. However, the induction of AP-1 activity by IL-1 and phorbol esters is dependent upon the presence of PKA, as evidenced by the loss of AP-1 inducibility in cells transfected with a cDNA encoding protein kinase inhibitor, a specific inhibitor of PKA. The effect of protein kinase inhibitor on AP-1 activation in response to IL-1 and tetradecanoyl-phorbol-13-acetate was reversed in the presence of the serine/threonine protein phosphatase inhibitor okadaic acid. Thus, the level of AP-1 activity in T cells may be determined by the balance between the activities of several serine/threonine protein kinases and phosphatases.