If bone marrow transplantation is to become widely applicable in the treatment of patients with leukemia and aplastic anemia, the necessity to have a perfectly histocompatible donor must be overcome. In an effort to define the roles of HL-A type and mixed lymphocyte culture (MLC) reactivity in the determination of successful engraftment and the occurrence of graft vs. host disease (GVHD), transplantation of a child with acute myeloblastic leukemia (AML) was attempted using an HL-A identical, MLC-reactive sibling donor. Successful engraftment was accomplished, as documented by the appearance of multiple donor genetic markers in the recipient. There is no evidence of severe GVHD. The recipient is alive, without evidence of leukemia and has returned to full activities 9 mo. after transplantation. The recipient now produces lymphocytes, which have the MLC reactivity that characterize the donor''s lymphocytes, rather than that of her own pretransplant lymphocytes. This experience demonstrates that successful bone marrow transplantation patients with leukemia can be accomplished in the face of MLC reactivity.