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.

Inclusion Criteria:

• Histologically confirmed diagnosis of low- grade NHL or transformed low-grade NHL (tumor must be CD 20 positive).
• Prior treatment with at least one chemotherapy regimen and have relapsed or progressed, or failed to achieve an objective response on last chemotherapy regimen.
• Karnofsky performance status of at least 60% and anticipated survival of at least 3 months.
• Absolute granulocyte of >/= 1,500/mm3.
• Platelet count of >/= 100,000/mm3, and not require sustained support of hematopoietic cytokines, or transfusion of blood products.
• Adequate renal function (i.e., <1.5x Upper Limit of Normal), and hepatic transaminases (AST <5 times ULN).
• Signed IRB/IEC-approved informed consent.

Exclusion Criteria:

• Patients with a mean of >25% of the intratrabecular marrow space involved with lymphoma.
• Patients who received cytotoxic chemotherapy, radiation therapy, immunotherapy, or cytokine treatment within 4 weeks prior to study entry (6 weeks for nitrosurea compounds) or who exhibit persistent clinical evidence of toxicity.
• Patients who have undergone stem cell or bone marrow transplant, active obstructive hydronephrosis, active infection, New York Heart Association Class III or IV heart disease or other serious illness that would preclude evaluation.
• Known HIV infection.
• Pregnant or nursing patients.
• Patients with prior malignancy other than lymphoma, except for adequately-treated skin cancer, in-situ cervical cancer, or cancer for which the patient has been disease-free for 5 years.
• Patients with progressive disease within 1 year of irradiation arising in a field that has been previously irradiated with more than 3500 cGy.
• Patients who received prior radioimmunotherapy, known brain or leptomeningeal metastases, HAMA positivity.
• Patients who are receiving either approved or non-approved (through another protocol) anti-cancer drugs or biologics.