Activin, a member of the transforming growth factor (TGF)? family of growth factors, is known to be a potential growth inhibitor in various tissues and a potential regulator of inflammation by downregulation of proinflammatory cytokines, like interleukin (IL)-1 and IL-6. Activin is overexpressed in several diseases, e.g. in the synovium of rheumatoid arthritis, which is characterized by inflammation and hyperplasia. Intracellular signaling of cellular growth arrest mediated by activin involves Smad proteins, but other molecular pathways still have to be clarified. The aim of this study was to characterize the intracellular signaling of activin that leads to cell growth inhibition.