Graft-versus-host disease (GVHD) is a common complication following bone marrow transplantation. Activation of the P2X7 receptor by extracellular ATP, which is released during immune responses and upon tissue damage, has been implicated in an allogeneic mouse model of GVHD. In this study, we investigated the effect of P2X7 blockade in a humanised mouse model of GVHD. Humanised mice, where NOD-SCID-IL2Rγnull mice (NSG) are injected with human (h) PBMCs, demonstrate GVHD 6-8 weeks post-injection. P2X7 expression in NSG mice was demonstrated using quantitative real-time PCR and Western Blotting. Brilliant Blue G (BBG), a specific P2X7 antagonist, exhibited a concentration-dependant blockade of both human and murine P2X7 in vitro. Humanised mice were injected with BBG (50 mg/kg) or saline (control) (Days 0,2,4,6 and 8), to examine the effect of P2X7 blockade on the incidence and severity of GVHD in this model. Flow cytometric analysis indicated that BBG did not affect engraftment of hPBMCs, predominantly hCD3+ T cells, in the blood at three weeks post-injection. Both BBG-injected, and control mice developed GVHD. Humanised mice showed engraftment of a higher proportion of hCD4+ T cells than hCD8+ T cells in the spleen, however there was no difference between treatments. This study demonstrated the presence of IFN-γ in both splenic hCD4+ and hCD8+ T cells, with higher amounts of IFN-γ in hCD8+ T cells; but no difference in cellular IFN-γ amounts between treatments. However, the concentration of hIFN-γ in serum of humanised mice treated with BBG was lower compared to that of control mice. These results indicate that although BBG could not prevent the development of GVHD, the effect on IFN-γ production warrants further research into P2X7 blockade as a potential therapy for GVHD.