Biliary tract malignancies represent challenges because of the lack of effective therapy and poor prognosis, in part because of the paucity of information regarding the mechanisms regulating their growth. We have recently identified a critical role for the p44/p42 mitogen-activated protein kinase (MAPK) pathway in interleukin 6 (IL-6)-stimulated growth of human cholangiocytes. Although IL-6 is a potential mitogen for cholangiocarcinoma, the role of this cytokine and its intracellular signaling pathways in cholangiocarcinoma growth is unknown. Thus, our aims were to determine the role of IL-6-mediated signaling mechanisms, and in particular the MAPK pathways, in the growth regulation of human cholangiocarcinoma. KMCH-1 cells (malignant cholangiocyte cells) secreted IL-6 constitutively, and increased IL-6 secretion in response to inflammatory cytokines such as tumor necrosis factor alpha (TNF-alpha) and IL-1beta. Stimulation with IL-6 resulted in proliferation of malignant cholangiocytes. These cells also possessed the IL-6 receptor complex subunits as directly assessed by immunoblot analysis. Furthermore, proliferation was completely inhibited by preincubation with anti-IL-6 neutralizing antibodies, indicating that the proliferative response to IL-6 involved receptor-mediated signaling. Both p38 and p44/p42 MAPKs were constitutively present and active in malignant cholangiocytes, and increased activity of both was observed within 15 minutes of stimulation with IL-6. Selective inhibition of either the p44/p42 MAPK pathway, by PD098059, or of the p38 MAPK pathway, by SB203580, blocked proliferation in response to IL-6. Thus, IL-6 can contribute to the autocrine and/or paracrine growth stimulation of malignant cholangiocytes via activation of either p38 or p44/p42 MAPK signaling pathways.