A Phase III, Randomized, Double Blind, Placebo-Controlled Study to Evaluate the Efficacy and Safety of Prochymal(Ex Vivo Cultured Adult Human Mesenchymal Stem Cells) Infusion for the Treatment of Steroid Refractory Acute GVHD

Inclusion Criteria:

- Subjects must be 6 months to 70 years of age, inclusive

- Subjects who have failed to respond to steroid treatment: Failure to respond to
steroid treatment is defined as any grade B-D (IBMTR grading) of acute GVHD that
shows: No improvement after 3 days and a duration of no greater than 2 weeks while
receiving treatment with Methylprednisolone (greater than or equal to 1 mg/kg/day) or
equivalent.

- Subjects must be treated within 4 days of randomization . In urgent situations 2nd
line therapy may be started 24 hours prior to randomization , and Prochymal must be
initiated within the following 3 days.

- Subjects who have received an increase in their steroid dose treatment prior to
randomization will be eligible for enrollment. An increase in steroid dose will not
be considered as second line therapy.

- Subjects must have adequate renal function as defined by: Calculated Creatinine
Clearance of >30mL/min using the Cockroft Gault equation

- For pediatric patients: Schwartz equation: (Patient population: infants over 1 week
old through adolescence (<18 years old)

- Subjects who are women of childbearing potential, must be non-pregnant, not
breast-feeding, and use adequate contraception. Male subjects must use adequate
contraception

- Subject must have a minimum Karnofsky Performance Level of at least 30 at the time of
study entry

- Subject (or legal representative where appropriate) must be capable of providing
written informed consent.

Exclusion Criteria:

- Subject has started treatment with second line therapy >24 hours prior to
randomization.

- Subject has received agents other than steroids for primary treatment of acute GVHD

- Subject is participating in the CTN Protocol 0302

- Subject has any underlying or current medical or psychiatric condition that, in the
opinion of the Investigator, would interfere with the evaluation of the subject
including uncontrolled infection, heart failure, pulmonary hypertension, etc.

- Subjects may not receive any other investigational agents (not approved by the FDA)
concurrently during study participation or within 30 days of randomization.

- Subject has a known allergy to bovine or porcine products.

- Subject has received a transplant for a solid tumor disease.