Interleukin-6 (IL-6) is known to be a major mediator of the acute-phase response in liver. We show here that IL-6 triggers the rapid activation of a nuclear factor, termed acute-phase response factor (APRF), both in rat liver in vivo and in human hepatoma (HepG2) cells in vitro. APRF bound to IL-6 response elements in the 5'-flanking regions of various acute-phase protein genes (e.g., the alpha 2-macroglobulin, fibrinogen, and alpha 1-acid glycoprotein genes). These elements contain a characteristic hexanucleotide motif, CTGGGA, known to be required for the IL-6 responsiveness of these genes. Analysis of the binding specificity of APRF revealed that it is different from NF-IL6 and NF-kappa B, transcription factors known to be regulated by cytokines and involved in the transcriptional regulation of acute-phase protein genes. In HepG2 cells, activation of APRF was observed within minutes after stimulation with IL-6 or leukemia-inhibitory factor and did not require ongoing protein synthesis. Therefore, a preexisting inactive form of APRF is activated by a posttranslational mechanism. We present evidence that this activation occurs in the cytoplasm and that a phosphorylation is involved. These results lead to the conclusions that APRF is an immediate target of the IL-6 signalling cascade and is likely to play a central role in the transcriptional regulation of many IL-6-induced genes.