High-dose chemotherapy followed by autologous stem cell transplantation (ASCT) has become the standard of care for relapsed/refractory chemotherapy-sensitive non-Hodgkin's lymphomas (NHL). However, one-half to two-thirds of such patients will relapse after ASCT, with subsequent poor prognosis, and new therapies are urgently needed for this patient population. Radioimmunotherapy (RIT) as a single agent therapy in patients with CD20 antigen-expressing relapsed or refractory low-grade, follicular, or transformed NHL has demonstrated overall response rates of 60-80% and has been approved by the FDA for use in this setting. While RIT is currently under investigation as a component of conditioning regimens for ASCT, the safety and efficacy of RIT after ASCT has not yet been well described. We will conduct a single-center Phase I dose-escalation trial of Bexxar (Tositumomab and 131I Tositumomab) for treatment of relapsed or residual CD20 antigen-expressing B-cell lymphomas following ASCT. Our primary aim will be to determine the safety, dose-limiting toxicity, and maximum tolerated dose of Bexxar in this post-ASCT patient population. Our secondary aim will be to describe the overall response rate, progression-free survival, time to treatment failure, and overall survival. Should Bexxar prove to be safe in this population, subsequent trials will be designed to investigate further the efficacy of RIT in the post-transplant setting.