An immunotoxin specific for human T lymphocytes was prepared by coupling an IgG2a anti-CD3 murine monoclonal antibody (64.1) to purified ricin A chain (64.1-A). Treatment of blood mononuclear cells with this immunotoxin at a concentration of 1.7 X 10(-9) mol/L for two hours at 37 degrees C in the presence of 20 mmol/L NH4Cl decreased phytohemagglutinin-stimulated protein synthesis by 95%. In addition, a sensitive culture assay showed that fewer than 0.03% T cells remained after treatment of human bone marrow mononuclear cells with 64.1-A at a concentration of 1.7 X 10(-9) mol/L. The inhibition of protein synthesis could be prevented by preincubating cells with unconjugated 64.1 antibody but not by preincubating cells with a control IgG2a antibody that binds to a different T cell antigen (CD5). At concentrations up to 1 X 10(-8) mol/L, 64.1-A had little effect on blood mononuclear cells from baboons or human myeloid precursors (CFU- GM), which do not express the CD3 antigen recognized by 64.1. Taken together, these results indicate that the toxicity of 64.1-A was specific and that 64.1-A may be a useful reagent for depleting T cells from donor marrow as a means of preventing acute graft-v-host disease after allogeneic bone marrow transplantation.