The major purpose of this study is to evaluate the way the body uses and absorbs (the pharmacokinetic profile) a drug called anti tumor necrosis factor antibody (infliximab) for the treatment of acute GVHD. Infliximab is currently indicated for the treatment of immunologic-based diseases (rheumatoid arthritis, moderately to severely active Crohn's disease, and fistulizing Crohn's disease), assuming patients have had inadequate responses to conventional therapy. It is not approved for the treatment of GVHD.