Inclusion Criteria
•A 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).
•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 a platelet count >= 100,000/mm3.
•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).
•Human Anti-Murine Antibody (HAMA) negative.

Exclusion Criteria

•Greater than 25% of the intratrabecular marrow space involved by lymphoma in bone marrow biopsy specimens.
•Hypocellular bone marrow (<=15% cellularity or marked reduction in bone marrow precursors).
•Prior myeloablative therapy.
•History of failed stem cell collection.
•Prior radiotherapy to fields encompassing more than 25% of the blood-forming marrow.
•Prior chemotherapy, biologic therapy, radiation therapy, or steroid therapy for non-Hodgkin's lymphoma (NHL) within eight weeks prior to screening procedures. Patients receiving low doses of steroids for non-neoplastic indications may enter the study (“low dose steroids” is defined as <=10 mg of prednisone or equivalent per day.)
•Prior radioimmunotherapy.
•Prior treatment of any non-human, particularly murine monoclonal or polyclonal antibodies for either diagnostic or therapeutic purposes. This prohibition does not extend to the chimeric monoclonal antibody, Rituximab.
•Prior malignancy other than lymphoma, except for adequately treated basal cell or squamous cell skin cancer, in situ cervical cancer, or other cancer for which the patient has been disease-free for five years.
•Evidence of active infection requiring IV antibiotics at the time of study enrollment.
•New York Heart Association (NYHA) class III or IV heart disease or other serious illness that would preclude evaluation.
•Known HIV infection.
•Known brain or leptomeningeal metastases.
•Active obstructive hydronephrosis.
•Evidence of ascites observed at screening physical examination.
•Known type I hypersensitivity or anaphylactic reactions to murine proteins or to any component of the Ibritumomab Tiuxetan therapeutic regimen, including Rituximab, Ibritumomab Tiuxetan, Yttrium Chloride, and Indium Chloride, or any component of the Iodine I 131 Tositumomab therapeutic regimen.
•Pregnant or nursing patients.