Transplantation of T-cell-depleted C57BL/6-Nu/Nu (“nude”) bone marrow (BM) into C3H/HeJ recipients, conditioned with 8 Gy total body irradiation plus chemotherapy with the myeloablative drug dimethyl myleran, resulted in poor hematopoietic reconstitution 14 days posttransplant, compared with transplantation with T-cell-depleted BM from normal C57BL/6 donors. Hematopoietic reconstitution of “nude” BM could be improved by the addition of (C57BL/6xC3H/HeJ)F1 thymocytes void of graft-versus-host activity. Enhancement of BM allografting by thymocytes is sensitive to low radiation doses (> or = 5.0 Gy) and can be achieved by transplanting the BM 24 hours before the administration of thymocytes. Fractionation of F1 thymocytes by differential agglutination with peanut agglutinin (PNA) and by fluorescence activated cell sorting showed that this hematopoietic enhancing activity is enriched in the unagglutinated (PNA-) thymocyte fraction and is mediated by PNA- CD8+ and not by PNA- CD4+ thymocytes.