Effects of protein-bound polysaccharide (PS)-K on interleukin (IL)-2-induced responses of peripheral blood mononuclear cells (PBMCs) were studied. PS-K (50 mcg/ml) was observed to enhance proliferative responses, cytotoxic activities against K562 and Daudi target cells, CD25+ cell population and telomerase activity of PBMCs stimulated with IL-2. The cytotoxic effector cells could be generated in the presence of PS-K even with a minimum amount of IL-2. The enhancing effect of PS-K on the IL-2-induced lymphocyte activation was more evident in PBMCs from cancer patients than in those from healthy volunteers, suggesting that PS-K may be beneficial if combined in the IL-2-based immunotherapy of cancer. TGF-beta inhibited the IL-2-induced lymphocyte activation of proliferative responses, cytotoxic activities and CD25+ cell population, the inhibitions of which were abrogated with PS-K. PS-K also abrogated the TGF-beta-induced anchorage-independent growth of normal rat kidney cells. Flow cytometric analysis using a labeled TGF-beta revealed that PS-K blocked the binding of TGF-beta at its receptor level on the surface of PBMCs. It is suggested that PS-K enhances IL-2-induced lymphocyte activation through, in part, an antagonistic action against TGF-beta.