This study has been cancelled prior to enrollment

This is a multi-center, randomized, study to compare Iodine I 131 Tositumomab therapeutic regimen to Ibritumomab Tiuxetan therapeutic regimen in the treatment of patients with relapsed or transformed follicular non-Hodgkin's B-cell lymphoma. A total of 350 patients, approximately 175 patients per arm, will be enrolled at 30 to 40 sites in the United States.

Inclusion criteria:

• Histologically confirmed diagnosis of follicular lymphoma, Grade 1, 2, or 3, or diffuse large cell lymphoma concurrent with or following the diagnosis of follicular lymphoma (WHO/REAL classification).
• International Working Formulation histological equivalents of Follicular, small cleaved; Follicular, mixed small-cleaved and large-cell; follicular large-cell; or Transformed diffuse large cell lymphoma following or concurrent with a diagnosis of follicular lymphoma.
• Patients diagnosed with diffuse large cell lymphoma at study enrollment must have a historical or contemporaneous lymph node biopsy that demonstrates a diagnosis of follicular lymphoma.
• Recurrent lymphoma after at least three qualifying therapy regimens including at least one Rituximab-containing regimen and at least one chemotherapy regimen.
• The patient must have either not responded or responded with a duration of response of less than 6 months to a Rituximab-containing regimen, Performance status of at least 70% on the Karnofsky Scale and an anticipated survival of at least three months.
• Bi-dimensionally measurable disease with at least one lesion measuring 4.0 cm2 by CT scan.
• Absolute neutrophil count >/= 1500 cells/mm3 and platelet count >/=100,000/mm3 within 21 days prior to study enrollment.
• Blood products and/or growth factors should not be taken within 4 weeks prior to blood draw.
• Adequate renal function (defined as serum creatinine <1.5 x upper limit of normal) and adequate hepatic function (defined as total bilirubin <1.5x upper limit of normal and AST <5x upper limit of normal) within 21 days prior to study enrollment.
• Human Anti-Murine Antibody (HAMA) negative within 21 days prior to study enrollment.
• Provision of informed consent as signified by a signed IRB approved consent form prior to any study-specific procedures being implemented.

Exclusion criteria:

• Greater than 25% of the intratrabecular marrow space involved by lymphoma in bone marrow biopsy specimens as assessed microscopically within 90 days prior to study enrollment.
• Hypocellular bone marrow (\