Study of Safety and Efficacy of a Sequential Regimen Consisting of Three Cycles of Fludarabine Followed by Tositumomab and Iodine I 131 Tositumomab

Inclusion Criteria:

* Patients must be age 18 years or older.
* Patients must have a histologically-confirmed diagnosis of low-grade or follicular non-Hodgkin's B-cell lymphoma.
* Patients must have Ann Arbor stage III or IV extent of disease after completing staging.
* Patients must have bi-dimensionally measurable disease. At least one lesion must have both perpendicular diameters \> 2 cm.
* Patients must have evidence that their tumor expresses the CD20 antigen by immunohistochemistry or flow cytometry.
* Patients must have no previous treatment for NHL.
* Patients must have a Karnofsky performance status of at least 60% and an anticipated survival of at least 3 months.
* Patients must have absolute granulocyte count greater than or equal to 1500 cells/mm3 and a platelet count \> 100,000 cells/mm3 within 14 days of study entry and not require sustained support with hematopoietic cytokines or transfusion of blood products.
* Patients must have adequate renal and hepatic function.
* Patients must sign IRB approved informed consent form(s) prior to study entry.

Exclusion Criteria:

* Patients who received systemic steroids within 1 week of study entry, except patients on maintenance steroid therapy for a non-cancerous disease.
* Patients with evidence of active infection requiring intravenous antibiotics at the time of study entry.
* Patients with New York Heart Association class III or IV heart disease or other serious illness that would preclude evaluation.
* Patients with known HIV Infection.
* Patients with known brain or leptomeningeal metastases.
* Patients who are pregnant or nursing. Patients of childbearing potential must undergo a pregnancy test at screening and on the day fludarabine treatment is started. Treatment is not to be administered until a negative result is obtained. Males and females must agree to use effective contraception for 6 months following the iodine I 131 tositumomab therapy.
* Patients with prior malignancy other than lymphoma, except for adequately-treated skin cancer in situ cervical cancer, or other cancer for which the patient has been disease-free for 5 years.
* Patients with hypersensitivity to fludarabine.
* Patients who are receiving either approved or non-approved (through another protocol) anti-cancer drugs or biologics.
* Patients who are HAMA positive.
* Patients with previous allergic reaction to iodine. This does not include reacting to intravenous iodine containing contrast materials.

Inclusion Criteria for Iodine I 131 Tositumomab Therapy

* Patients who completed 3 cycles of fludarabine.
* Patients must have absolute granulocyte count ≥ to 1500/mm3, platelet count of ≥ 100,000/mm3 (≥ 150,000/mm3 if \> 25% bone marrow involvement at restaging), and not require sustained support with hematopoietic cytokines or transfusions with blood products.
* Patients must have adequate renal and hepatic function.

Exclusion criteria for Antibody Therapy

* Patients with active obstructive hydronephrosis.
* Patients with evidence of active infection requiring intravenous antibiotics.
* Patients who are pregnant.