Graft-versus-host disease (GVHD) induced by transplant-derived T cells represents a major complication after allogeneic bone marrow transplantation (BMT). However, these T cells support engraftment, early T-cell immunity, and mediate the graft-versus-tumor (GVT) effect. Cytotoxic effector functions by transplanted T cells are predominantly mediated by the perforin/granzyme and the CD95/CD95L system. APG101, a novel recombinant human fusion protein consisting of the extracellular domain of CD95 and the Fc domain of an IgG1 antibody inhibited CD95L-induced apoptosis without interfering with T-cell function in vitro and was therefore tested for its ability to prevent GVHD in murine BMT models across minor or major histocompatibility barriers. Starting APG101 treatment either 1 day before or 6 days after transplantation effectively reduced clinical GVHD and rescued survival between 60% and 100% if GVHD was CD95L mediated. APG101 did not interfere with the GVT effect, because P815 mastocytoma and most importantly primary Bcr-Abl-transformed B-cell leukemias were completely eradicated by the alloantigen-specific T cells. Phenotype and homing of alloantigen-specific T cells or their perforin/granzyme-mediated cytotoxicity and proliferative capacity were not affected by APG101 treatment suggesting that APG101 therapy might be useful in GVHD prophylaxis without impairing T-cell function and most importantly preserving GVT activity.