A non-complement-binding monoclonal antibody, TA-1, recognizing determinants on human T lymphocytes, was linked to the plant seed toxin ricin, either the intact molecule or purified ricin A chain. Peripheral blood lymphocytes were pretreated with conjugate for 2 hr, washed, and then measured in vitro for T cell proliferation. Studies showed that antibody-intact ricin conjugates were up to 39-fold more inhibitory than antibody-A-chain conjugates. Killing was selective because an unreactive control antibody linked to toxin had minimal inhibitory effect. Dose response curves obtained in human studies were nearly identical to curves obtained in an animal model n which a monoclonal anti-murine T cell antibody (anti-Thy 1.1) was linked to ricin and ricin A chain. In the human system, longer exposures of peripheral blood cells to conjugates did not alter our findings. TA-1-ricin conjugates were tested against human ALL cell lines. KOPN-1, a cell line bearing the determinant reactive with TA-1 was selectively eliminated within 2 days after pretreatment with 500 ng/ml. Even 10-fold greater concentrations of TA-1-A chain were not adequate for leukemic cell destruction. These findings (1) show for the first time in a human model that monoclonal antibodies, directed against certain differentiation antigens when linked to ricin A chain are not as effective in normal or malignant cell killing as when linked to intact ricin; (2) contribute to the growing body of evidence showing that monoclonal antibody A chain conjugates do not permit the acquisition of levels of toxin sufficient to destroy target cells; and (3) are important relative to increasing interest in use of antibody-toxin conjugates for graft-versus-host disease prophylaxis in allogeneic bone marrow transplantation.