Bone marrow transplantation was considered effective treatment for leukemias and other haematological diseases. Acute graft v-host disease (GVHD) is a major complication of allogeneic bone marrow transplantation. The T lymphocytes were specially depleted by anti-CD5, CD2, CD8, CD27 monoclonal antibody-ricin immunotoxins. Inhibition of the protein synthesis and T-cell functions in the target cells was observed in vitro. At 10 9mol / L of the immunotoxin, 3 logs (99.9%) of target cells were killed, but no cytotoxicity on nontarget cells. At the same concentration, the anti T cell immunotoxin had no any influence on the proliferating rate of CFU-GM and BFU E. None of the ten patients who received T cell depleted bone marrow developed grade III or IV acute GVHD.