Background/Aims: The growth of normal hepatocytes is regulated by the activin-follistatin system. The aim of this study was to investigate the activin-follistatin system in hepatoma cells. Methods: The production and action of activin and follistatin in human hepatoma cell lines were examined. Activin A and follistatin were measured by bioassay and protein-binding assay, respectively. Results: Activin A inhibited cell growth in HepG2 cells but not in either PLC/PRF/5 or HLE cells. However, the effect of activin A in HepG2 cells was attenuated at high cell density. In HepG2 cells, two classes of activin-binding sites were expressed, and affinity cross-linking showed that 125I-activin A bound specifically to three proteins with molecular weights of 48, 67, and 94 kilodaltons. In PLC/PRF/5 cells, a single class of binding site was observed, and the binding capacity was approximately 60% of the capacity in HepG2 cells. Virtually no 125I-activin A binding was detected in HLE cells. Bioactivity and messenger RNA for activin A were undetectable in three cell lines. In contrast, follistatin was released from three cell lines. Conclusions: Multiple alterations in the activin-follistatin system were found in three hepatoma cell lines. The accelerated growth observed in hepatoma cells may be caused, at least partly, by the attenuation of the action of activin A.