We hypothesized that radiolabeled anti-CD20 antibody will be more efficacious than unlabeled antibody (i.e. rituximab) in eradication of highly radiosensitive myeloma cells, similarly to what was observed in non-Hodgkin's lymphoma (Horning et al., 2000; Davis et al, 2001 Flinn et al, 2001; Witzig et al, 2002). It is presumed, that higher efficacy of radiolabeled anti-CD20 antibody, if compared to unlabeled antibody, may be due to "crossfire effect" as the radiation is delivered not only to target cells but also to neighboring cells, both expressing and not-expressing CD20 (Vose et al, 1999). In the proposed design, we will apply Bexxar after completion of a course of initial chemotherapy for newly diagnosed patients or a course of salvage chemotherapy, provided that patients achieve at least partial response to prior therapy (i.e. > 50% reduction of tumor mass) and reach a plateau of their response to prior therapy. By using patients in a period of stable disease, we will have a better chance to observe the effects of radioimmunotherapy against CD20+ cells, which represent a minority of all malignant cells. In addition, we will evaluate the impact of Bexxar therapy on clonogenic myeloma cells using clonogenic assays as described in Methods.