Allogeneic stem cell transplantation (allo SCT) is now frequently performed for the treatment of haematological malignancies and aplastic anaemia. However, graft-versus-host disease (GVHD) is still the major complication after allo SCT, producing immune deficiency, infection, organ damage and, occasionally, patient death. The antigen-specific signal mediated by the T-cell receptor (TCR) is essential for activation of T-cells; however, additional co-stimulatory signals are required for complete T-cell activation. Therefore, blocking strategies of co-stimulatory signals have been evaluated as targets of therapeutic intervention for GVHD after allo SCT. In a mouse bone-marrow transplantation (BMT) model, the administration of CTLA4-Ig, which blocks the interaction of CD28 on T-cells and B7 molecules on antigen-presenting cells (APCs), can prolong survival of allo BMT recipients, although this effect was not complete. In addition, the anti-CD40L (CD154) monoclonal antibody (mAb), which can interfere with the interaction of CD154 on T-cells and CD40 on APCs, can induce long-term graft survival in the murine model. Combined administration of CTLA4-Ig and anti-CD40L mAb can prevent allograft rejection in primates. Therefore, it seems the most powerful method to prevent the alloimmune response in vivo. The Fas/Fas ligand pathway is also involved in pathogenesis of GVHD. Anti-FasL mAb can reduce the mortality of GVHD and improve intestinal lesions. Recently, it was reported that donor bone marrow treated ex vivo using CTLA4-Ig reconstituted haematopoiesis in vivo with a relatively low risk of GVHD in human allo BMT. Therefore, selective blocking strategies for T-cell co-signalling might be useful for the prevention of GVHD in human allo SCT.