Inclusion Criteria:

- Allogeneic bone marrow or hematopoietic stem cell graft recipients

- Acute GVHD of at least grade II severity

- Failure of GVHD to improve on at least 2 mg/kg/day of methylprednisolone or
prednisone for at least three days or recurrence of acute GVHD as corticosteroids are
tapered from initial treatment of the initial episode of GVHD

- Evidence of engraftment (ANC over 1000 cells/mm3)

- Histologic evidence of GVHD from biopsy performed during the current episode

- Received GVHD prophylaxis of methotrexate, tacrolimus or cyclosporine

- Age at least 18 years

- Body weight under 130 kg

- Both males and females are eligible but females of childbearing potential will use an
accepted method of avoiding pregnancy for at least 60 days after the end of treatment
(which includes oral or implanted contraceptives, IUD, female condom, diaphragm with
spermicide, cervical cap, abstinence, use of a condom by the sexual partner or a
sterile sexual partner)

Exclusion Criteria:

- Previous receipt of MEDI-507

- Clinical or histologic manifestation of chronic GVHD

- Previous treatment with any anti-T-cell antibodies such as OKT®3 or daclizumab
(Zenapax®)

- Receipt of antithymocyte globulin (ATGAM® or other ATG) since the day of transplant

- More than one allogeneic bone marrow or hematopoietic stem cell allograft

- Moribund and unlikely (in the opinion of the investigator) to survive 15 days

- Use of other investigational agents within 30 days (this does not include the use of
licensed agents for indications not listed in the package insert)

- Any of the following clinical settings or diagnoses:

Ø documented or presumed significant active infection Ø pregnancy or nursing mother Ø
evidence of infection with HIV-1, hepatitis B or C virus Ø hemodialysis or chronic
peritoneal dialysis Ø use of a ventilator

- Histologically confirmed veno-occlusive disease of the liver

- Ascites on physical examination (this does not include small amounts of ascitic fluid
detected only on ultrasound)