Seventeen patients, ages 7–53 years were transplanted with histocompatible bone marrow that had been depleted of Τ lymphocytes by ex vivo immunotoxin (IT) treatment. Twelve patients had high-risk acute leu-kemias, and five had chronic myelogenous leukemia. No other graft-vs.-host disease (GVHD) prophylaxis was used. A mixture of three anti-T-cell monoclonal antibodies conjugated to ricin were used in this study: TA-1, UCHT-1 (anti-CD3), and T101 (anti-CD5). The mean number of bone marrow cells infused was 1.5×10s mononuclear cells/kg recipient weight. Thirteen of the 17 patients demonstrated complete and sustained en-graftment. Four patients experienced autologous marrow recovery and/or graft rejection. Compared with an historical group of leukemic patients who received GVHD prophylaxis with methotrexate alone or combinations of methotrexate, and prednisone plus antithy-mocyte globulin, (ATG) or OKT3, the IT patients with stable engraftment demonstrated shorter time to recovery of leukocytes ≥ 1000mm3 for three consecutive days (median, 20 days vs. 26 days, P=.03). The recovery of total lymphocytes, Β and Τ cell subsets, and Τ cell function by day 28 was highly variable, but similar, for patients in both the IT-treated group and historical controls. Four patients (ages 13, 18, 21, and 38) developed grade II skin GVHD, but none had severe GVHD. Eight of the 13 patients with durable engraftment have had posttransplant leukemic relapse. Currently only four patients remain alive; two have not relapsed posttransplant, while the other two achieved remission following posttransplant relapse. We conclude that severe GVHD was not observed in this small series with ex vivo Τ cell depletion for GVHD prophylaxis, and that favorable recovery of hematologic and lymphocytic function was demonstrated for cases where primary engraftment was sustained. A larger randomized controlled study will be needed to establish whether Τ cell depletion of donor bone marrow with IT can significantly reduce GVHD, and/or improve disease-free survival.