Recent technical advances in stem cell transplantation have made the use of HLA-haploidentical family donors an attractive option to reliance on MHC-identical donors. Although the problems of graft rejection and graft-versus-host disease have been largely overcome, this strategy has failed to secure significant gains in long-term survival, primarily because of lengthy delays in immune reconstitution and devastatingly high rates of lethal infection with opportunistic organisms. One alternative would be to infuse T cells that have been depleted of alloreactive lymphocytes, while retaining cells with specificity against pathogens. We propose to obtain such cells by culturing donor T cells with recipient EBV-transformed B cells (EBV-LCL), followed by treatment with an ablative anti-CD25 immunotoxin (RFT5). EBV-LCL are excellent antigen presenting cells and can be reproducibly generated from all donors, but lack many of the polymorphisms expressed on normal myelomonocytic cells and on leukemic blasts that may be targets for graft-versus-leukemia activity. By using EBV-LCL in this way, it should be possible to reproducibly eliminate alloreactive T lymphocytes from the transplant, while sparing T cells able to recognize tumor cells or infectious agents. In the Phase I trial proposed here, escalating doses of donor lymphocytes, depleted of host lymphoblastoid cell line (LCL)-reactive T lymphocytes by treatment with the RFT5 immunotoxin, will be administered to recipients of haploidentical stem cell transplants (Aim 1). The dose escalation plan specifies enrollment of approximately 15 patients, from 1 to 65 years of age, over a 2-year period. Both overall and disease-free survival, as well as the frequency of documented infections, will be determined at 100 days and 1 year post-transplantation. A retroviral gene-marking strategy, developed in the applicant's laboratory, will be used to assess the contribution of the infused T cells to immune reconstitution (Aim 2). All of the technologies and operating procedures needed to implement this clinical study are in place in the applicant's laboratory or in the Center in which the patients will be treated. Upon completion of this project, both the maximum safe dose and in vivo fate of the infused non-alloreactive T cells should be clear.