Comparison of tumor biopsy samples before and after adoptive T-cell therapy for EBV lymphoma in an 11-year-old boy who had undergone allogeneic bone marrow transplantation from a matched unrelated donor. The initial specimen had the histologic appearance of immunoblastic lymphoma (a; hematoxylin and eosin). Most of the cells reacted to staining for the CD20 B-cell marker (b), whereas a few were positive for the T-cell marker (c). After adoptive immunotherapy, the follow-up biopsy showed an infiltrate of smaller lymphocytes, the majority of which reacted with the T-cell marker (d through f).