Protocol 260 - Subjects will be randomized with equal probability to the treatment arms (2 million cells/kg of Prochymal or 8 million cells/kg of Prochymal) using a stratified block design. The stratification factor is acute GVHD grade. For the purpose of stratification, the GVHD grades are II and III-IV. Treatment with investigational agent was administered on study Days 1 and 4. Patients were followed for safety and efficacy until Day 28 after initiation of treatment with the investigational agent, or until withdrawal or death, whichever occurred first.