Compositions and methods are provided for prevention and clinical treatment of various forms of graft-versus-host disease (GVHD) by using inhibitors of adenosine deaminase (ADA). In particular, various formulations and dosing regimens of ADA inhibitors such as pentostatin are provided for the treatment of humans in vivo as well as for ex vivo conditioning of organ transplants in order to specifically suppress T-lymphocyte mediated immune responses while minimizing systemic toxicity of the drug.