Donor lymphocyte infusion: A type of therapy in which lymphocytes from the blood of a donor are given to a patient who has already received a stem cell transplant from the same donor. The donor lymphocytes may kill remaining cancer cells. Also known as a buffy coat fusion, donor lymphocyte infusion (DLI) is a form of adoptive immunotherapy used after hematopoietic stem cell transplantation. Formerly, the only treatment option that offered relapsed bone marrow transplant patients hope of a cure was another bone marrow transplant. However, the risk of serious, life-threatening complications after a second bone marrow transplant is great. Donor lymphocyte infusion is used to treat chronic myelogenous leukemia (CML) that has come back and myeloma. It is being studied in the treatment of other types of cancer.