Allogeneic progenitor cell transplantation provides an example of adoptive immunotherapy in oncology. The graft contains immmune competent cells that are responsible for a « graft versus leukemia » or « graft versus tumor » effect ; this contributes to the cure of malignancies, along with high-dose chemotherapy and radiotherapy. Infusion of donor immune-competent cells is also efficient to induce a remission, when a clinical or cytogenetical relapse occurs, following transplantation. The nature of effector cells and the mechanisms underlying the recognition of tumor cells are not fully understood. In allogeneic progenitor cell recipients, minor histocompatibility antigens are targets for immune-competent cells ; this explains why there is a clear relation between « graft versus leukemia » and « graft versus host disease » that limits the potential therapeutic applications of these procedures. The production of T cell clones with an anti-leukemic activity, that could be used in the setting of allogeneic or autologous transplantations, requires an improved knowledge of tumor antigens able to induce an immune reponse, and of mechanisms that create tolerance of tumor cells in cancer patients, and that will need to be overcome. Reports of the clinical use of T cell clones with anti EBV or anti CMV activity suggests that this approach is feasible.