In vitro studies of the mouse erythroleukemia cell system have identified at least 300 agents capable of inducing differentiation by mechanisms that remain to be elucidated. We have recently begun to examine recombinant cytokines as possible agents in inducing differentiation of tumor cells, specifically, malignant cells resistant to cytotoxic drugs. One such cytokine, transforming growth factor-beta (TGF-B1), is a multifunctional peptide that exists in at least five different isoforms in vertebrate species. Recently, there has been a great deal of interest in the role of TGF-B1 as an important multifunctional growth regulator that induces cells of mesenchymal origin to divide while inhibiting the growth of nontransformed epithelial cells. In this study, we combined the effects of the differentiation agent hexamethylene bisacetamide and the inhibiting effects of TGF-B1 on a multidrug-resistant human liver hepatocellular carcinoma and demonstrated the synergistic interaction of these two agents; this synergy resulted in a cell death rate of 80%. These data support the concept of programmed cell death and suggest that drug-resistant tumor cells may be susceptible to the combination of cytokines and differentiating agents.