Graft rejection in allogeneic bone marrow transplantation (BMT) can occur when donor and recipient are mismatched at one or more major histocompatibility complex (MHC) loci. Donor T cells can prevent graft rejection, but may cause fatal graft-versus-host disease (GVHD). We tested whether irradiation of allogeneic donor lymphocytes would preserve their graft-facilitating activity while inhibiting their potential for GVHD. Infusions of irradiated allogeneic T cells did not cause GVHD in MHC-mismatched SJL → (SJL × C57BL6) F1, C57BL6 → B10.RIII, and C57BL6 → B10.BR mouse donor → recipient BMT pairs. The 60-day survival among MHC-mismatched transplant recipients increased from 2% (BM alone) to up to 75% among recipients of BM plus irradiated allogeneic splenocytes. Optimal results were obtained using 50 × 10⁶ to 75 × 10⁶ irradiated donor splenocytes administered in multiple injections from day −1 to day +1. Recipients of an equal number of nonirradiated MHC-mismatched donor splenocytes uniformly died of acute GVHD. The graft facilitating activity of the irradiated allogeneic splenocytes was mediated by donor T cells. Irradiation to 7.5 Gy increased nuclear NFκB in T cells and their allospecific cytotoxicity. Irradiated T cells survived up to 3 days in the BM of MHC-mismatched recipients without proliferation. Recipients of irradiated allogeneic splenocytes and allogeneic BM had stable donor-derived hematopoiesis without a significant representation of donor splenocytes in the T-cell compartment. Irradiated allogeneic T cells thus represent a form of cellular immunotherapy with time-limited biologic activity in vivo that can facilitate allogeneic BMT without causing GVHD.