The purpose of this Phase I/II, open-label, dose-escalation study is to evaluate an investigational monoclonal antibody administered as a first-line therapy to patients with acute, Grade II, III, or IV graft-versus-host disease (GVHD). Patients will be eligible for enrollment within 24 hours of beginning standard steroid treatment. The research is being conducted at up to 10 clinical research sites in the US.