This is a single arm, multi-center, expanded access study of Iodine I 131 Tositumomab (BEXXAR) therapeutic regimen for patients with relapsed or refractory low-grade or transformed low-grade non-Hodgkin's B-cell lymphoma. The primary objective is to make Iodine I 131 Tositumomab more broadly available to patients. Secondary endpoints will be to obtain additional safety and efficacy information for this treatment regimen. Post study drug administration follow-ups will continue for up to ten years. These will include blood-work and adverse event assessments for 13 weeks post dosing, patient response evaluations at Week 13, Months 6, 12, 18, 24, and Long-Term Follow-ups every 6 months until the elapse of 5 years from the dosimetric dose and then annually thereafter through year 10. Thyroid function will be monitored annually during Long-term follow-up.