The transforming growth factor beta s (TGF beta) represent a family of multifunctional cytokines that modulate growth and function of many cells including those with malignant transformation. The actions depend on the tissue involved, and on the presence of signaling TGF beta receptors. Both in vitro and in vivo studies indicate that TGF beta mediates not only tumor progression but also exerts strong suppressive effects on certain tumor cells. Besides direct TGF beta - cell interactions, the modulation of immunocompetent cells by TGF beta may influence tumor cell growth. Further studies on the beneficial TGF beta effects on tumors, and even on chemotherapy including adverse reactions are required to define the use of TGF beta or TGF beta modulating agents in cancer therapy.