The influence of graft-versus-host (GVH) reaction on the host hematopoietic cells clinically manifests itself both as adverse reactions in transfusion-associated GVH disease (GVHD) and as a therapeutic graft-versus-leukemia (GVL) effect in either donor lymphocytes transfusion (DLT) or allogeneic bone marrow (BM) transplantation. We examined the effect of GVH reaction on the host hematopoiesis in the murine parent-into-F1 (P1 --> F1) model of GVHD. The systemic transfer of 5 x 10(7) of C57BL/6 (B6) splenocytes into (B6xDBA/2)F1 mice (BDF1), which results in acute GVHD, reduced the peripheral blood cell counts, the number of BM cells, and colony-forming unit-granulocyte macrophage (CFU-GM), whereas the injection of 10(8) of DBA/2 cells into BDF1, which results in chronic GVHD, did not affect hematopoiesis 2 weeks after the transfer. To clarify the mechanism of such myelosuppression, we examined the Fas expression in both hematopoietic progenitor cells as well as whole BM cells. The Fas expressions in each fraction significantly increased in BDF1 mice 2 weeks after the induction of acute GVHD, whereas no such effects were observed in the BDF1 mice with chronic GVHD. Furthermore, when such BM cells were incubated with anti-Fas antibody (Jo2), which induces apoptosis through Fas, the fraction of apoptotic cells increased and the number of CFU-GM decreased significantly. The in vivo administration of neutralizing anti-FasL antibody into BDF1 mice receiving with B6 spleen cells thus protected the host mice from BM failure. These results indicate that the functional expression of Fas on hematopoietic cells plays an essential role in the myelosuppressive effect of GVHD.