The human allogeneic mixed lymphocyte reaction is the in vitro correlate of graft rejection. Cytotoxic effector cells generated during an allogeneic mixed lymphocyte reaction were previously shown to express the human p55 IL-2 receptor subunit, whereas resting cells do not express this receptor peptide. In this study, we asked whether Pseudomonas exotoxin or bismuth-212 (an alpha-particle emitting radionuclide) coupled to the anti-IL-2 receptor mAb, anti-Tac, were able to selectively eliminate alloresponsive cells generated during an allogeneic mixed lymphocyte reaction. After assembly, anti-Tac immunoconjugates retained their binding integrity, specificity, and selectivity. Deletion of alloresponsive cells was shown by the removal of alloproliferating cells as assessed by quantitating cell recovery and by measurement of thymidine incorporation into newly synthesized DNA. Both toxin and radionuclide immunoconjugates eliminated established cytotoxic effector cells generated in an allogeneic mixed lymphocyte reaction, while leaving intact the PHA-inducible mitogenic response of the nonactivated cells. The addition of excess anti-Tac blocked all of the effects of these cytotoxic reagents. The therapeutic reagents in vitro were most effective when added just prior to the peak of the alloproliferative response, when receptor expression would be close to maximum. Thus, anti-Tac conjugated either with toxin or radionuclide is effective in vitro in specifically eliminating cytotoxic effector cells.