Patients with multiple myeloma scheduled for standard of care high dose melphalan and autologous stem cell transplant who have an incomplete response to induction treatment (i.e. with measurable disease) following peripheral blood stem cell collection are candidates for participation on this trial. 90Y Zevalin targets CD20 expressed on the surface of mature B-cells and is FDA approved for relapsed/refractory low grade lymphoma. This is a single arm, phase II safety and efficacy study of 90-Y Zevalin in multiple myeloma. Subjects will receive cold antibody (Rituximab 100mg/m2) followed by 5 mCi test dose of 111-In Zevalin on transplant day -21. Gamma camera images are obtained 48 to 72 hours after 111-In Zevalin to document appropriate / expected distribution of radiotracer. On transplant day -14, subjects will receive another cold antibody dose followed by 90Y Zevalin 0.4 mg/kg (max 32 mCi) as single dose. Subjects are admitted for transplant on day -3 and proceed with institution standard high dose melphalan (200mg/m2) on day -2. Subjects are followed for safety for 6 weeks after transplant.