Patients with newly diagnosed acute GVHD will be able to participate in the study. Patients will receive a single dose of infliximab in the clinic. Since this is a Phase I study, the patients will have blood samples drawn to measure the pharmacokinetics of the drug. A total of 16 blood samples will be drawn over 84 days. The samples will be labeled with a code, processed, frozen, and then sent in a batch to the PBMTC designated laboratory for testing. Patients will continue on any drugs they were getting for the prevention of GVHD. Additional doses of infliximab may be given. This decision will be based on the results of the blood testing.