The purpose of this phase II study is to evaluate an investigational monoclonal antibody for the treatment of glucocorticoid-refractory Graft Versus Host Disease (GVHD). Patients diagnosed with GVHD who have not responded satisfactorily to, or are intolerant of, treatment with standard agents will be considered for entry. Patients will be allowed to continue on their other immunosuppressive drugs at stable doses during the trial. The research is being conducted at up to 20 clinical research sites in the US.