Safety and Efficacy of Prochymal for the Salvage of Treatment-Refractory Acute GVHD Participants

Inclusion Criteria: * Participants must be 6 months to 70 years of age inclusive. * If female and of childbearing age, participants must be non-pregnant, not breast-feeding, and use adequate contraception. Male participants must use adequate contraception. * Participants must have Grade III-IV acute GVHD that has failed to respond to standard first and at least one second-line therapy. Biopsy for confirmation of both skin and gastrointestinal GVHD is not mandatory, but is recommended when feasible. Enrollment should not be delayed awaiting biopsy results. * Participants must have minimal renal function as defined by: Calculated creatinine clearance (CrCl) of \> 30 milliliters/minute (mL/min) using the Cockcroft-Gault equation. * Participant must provide written informed consent and authorization for use and disclosure of protected health information (PHI). Exclusion Criteria: * Participant has uncontrolled alcohol or substance abuse within 6 months of treatment. * Participant has any underlying or current medical or psychiatric condition that, in the opinion of the Investigator, would interfere with the evaluation of the participant (e.g., uncontrolled infection, right heart failure, pulmonary hypertension, etc.). * Participant has a clinically significant, unstable arrhythmia. * Participant has a known allergy to bovine or porcine products. * Participant is unwilling to sign consent form for the long-term follow-up study, protocol 271.