In the present investigation, we compared the immunosuppressive effects of genistein and CsA on anti-CD28 stimulated human T cell proliferation, IL-2 production, and IL-2R expression. Genistein, an isoflavanoid compound, is a specific protein tyrosine kinase inhibitor and inhibited the PMA plus anti-CD28 stimulated T cell proliferation. In contrast, proliferation of T cells stimulated with PMA plus anti-CD28 is resistant to the inhibitory effects of CsA. Similar results were obtained with IL-2 synthesis and IL-2R expression. PHA plus anti-CD28 or PMA plus anti-CD28-induced IL-2 synthesis was inhibited by genistein, and CsA, though it inhibited the PHA plus PMA-stimulated IL-2 synthesis, failed to have any effect on PMA plus anti-CD28-induced IL-2 synthesis. Genistein at the concentration that inhibited T cell proliferation and IL-2 synthesis also showed significant inhibitory effects on PMA plus anti-CD28 stimulated IL-2R expression while CsA had no effect on IL-2R from these cultures. Our data suggest that genistein is a powerful immunosuppressive agent, with no toxic effects on T cells, and has the potential for use in the prophylaxis and treatment of allograft rejection. Since genistein blocks the CsA-resistant pathway of T cell proliferation, the combined usage of these two agents may provide better immunosuppressive effect and a lesser degree of CsA-induced nephrotoxicity.