Antileukemic autotransplantation, an approach to eradicate by antibodies residual leukemia in the bone marrow taken during remission and regrafted in relapse, is proposed and is the basis of investigations on the elimination of leukemic cells transferred to syngeneic mice. Rabbit antiserum against mouse T [thymus-derived] cells killed over 99% of cells when incubated with 105 lymphocytes of a T cell AKR/J leukemia transferred subsequently to syngeneic hosts. Seventy-five per cent of the recipients of antibody-coated and complement-treated leukemic cells survived the observation period of 80 days. Recipients conditioned with 800 R and given syngeneic bone marrow mixed with 105 leukemic cells died within 11 days, while 40% survived the observation period of 100 days without leukemia if the leukemic bone marrow was preincubated with anti-T cell globulin (ATCG) and complement. A detrimental effect of ATCG on normal T cells could only be demonstrated in thymectomized, irradiated recipients of syngeneic ATCG-treated marrow. These mice did not reject H-2-incompatible skin grafts in contrast to the controls without ATCG or without thymectomy. ATCG thus inhibited leukemic lymphocytes while sparing stem cells for hemopoietic reconstitution. It did not interfere with the recovery of cellular immunity after bone marrow transplantation.