This is a prospective, open-label pilot study in which 4 doses of Rituximab are administered to patients who have developed SR-aGVHD following allogeneic hematopoietic transplant (AHT). The study is designed to determine the overall survival at 180 days after treatment with rituximab, and evaluates the safety and clinical response to rituximab in this study population. Study entry: Patients must enter study on or before day +100 posttransplant.