Multiple myeloma is nowadays often cured by stem cell transplantation whereby the donor provides a new blood system to the patient. T-cells present in this graft eliminate the residual malignant cells but can also attack the patient's healthy tissues. To prevent this, T-cells can be depleted from the graft and replaced by T-cells specific for the patient's malignant blood cells only. This treatment option is hampered by difficulties in obtaining sufficient numbers of these "good" T-cells. This project proposes to use recombinant immunotoxins that exhibit the same tasks as the T-cells but are easy to obtain in large quantities.