Humanized or chimeric monoclonal antibodies (MoAbs) directed against the interleukin-2 (IL-2) receptor α-chain, CD25, are promising immunosuppressive agents due to improved pharmacokinetic profiles and less toxicity. These MoAbs have been used effectively in preventing and/or treating rejection in solid organ transplantation and are currently under investigation for prevention/treatment of graft-versus-host disease (GvHD) in stem cell transplantation. We analysed the in vitro activities of the chimeric anti-CD25 MoAb basiliximab and the humanized anti-CD25 MoAb daclizumab in various test systems for alloimmune response and T cell activation in comparison to cyclosporin A (CsA) and prednisolone. Anti-CD3- and alloantigen-induced T cell proliferation were decreased significantly by the anti-CD25 MoAbs in a dose-dependent fashion. At a concentration of 10 ng/ml daclizumab and CsA synergistically decreased T cell proliferation of mixed lymphocyte cultures, whereas basiliximab showed only subadditive activity. Simultaneous addition of the anti-CD25 MoAbs and prednisolone did not result in combined activity. Addition of exogenous IL-2 completely overcame the inhibitory effect on T cell proliferation of both anti-CD25 MoAbs, but not that of CsA and prednisolone. Anti-CD25 MoAbs inhibited the generation of antigen-specific cytotoxic T lymphocytes in a limiting dilution assay, whereas they showed no effect on the cytolytic activity of established antigen-specific T cell clones. This in vitro study demonstrates strong immunosuppressive activity by both chimeric and humanized MoAbs against CD25. The combined activity with CsA justifies their early use for prevention rather than treatment of GvHD.