Patients with non-Hodgkin's lymphoma who previously responded with a duration of response of at least 3 months to Iodine-131 Anti-B1 Antibody therapy will undergo two phases of study. In the first phase, patients will receive a dosimetric dose of unlabeled Anti-B1 Antibody (450 mg) followed by Anti-B1 Antibody (35 mg) which has been radiolabeled with 5 mCi of Iodine-131. Whole body gamma camera scans will be obtained after the dosimetric dose and data from three imaging time points will be used to calculate a patient-specific dose to deliver the desired total body dose of radiotherapy. In the second phase, patients will receive the therapeutic dose of unlabeled Anti-B1 Antibody (450 mg) followed by 35 mg of Anti-B1 Antibody labeled with the patient-specific dose to deliver the desired whole body dose of radiation. Patients will be treated with thyroid blocking medication at least 24 hours prior to the first infusion and continuing for 14 days following the last infusion.

Tositumomab and Iodine I 131 Tositumomab

Complete response (CR) is defined as the complete resolution of all disease-related radiological abnormalities and the disappearance of all signs and symptoms related to the disease. Response is defined as the best response achieved at any evaluation. A confirmed response is defined as a response that was confirmed by two separate response evaluations occurring at least 4 weeks apart.

For participants with CR, clinical CR (CCR), or partial response (PR), duration of response is defined as the time from the first documented response to the first documented progression. CCR is defined as the complete resolution of all disease-related symptoms, but residual foci, thought to be residual scar tissue, are present. Generally, an unchanging lesion <=2 centimeters (cm) in diameter by radiographic evaluation or <=1 cm in diameter by physical examination can be considered scar tissue. The extent of disease must be unchanged or decreased upon follow-up evaluations and, if unchanged or if further decreases for 6 months or longer are present, the participant will then be reclassified as a CR (complete resolution of all disease-related radiological abnormalities and the disappearance of all signs and symptoms related to the disease). PR is defined as a >=50% reduction in the sum of the products of the longest perpendicular diameters of all measurable lesions, with no new lesions.

Progression-free survival (time to progression or death) is defined as the time from the start of retreatment (i.e., the dosimetric dose) to the first documented progression or death. Disease Progression (PD) is defined as a >=25% increase from the nadir value of the sum of the products of the longest perpendicular diameters of all measurable lesions or the appearance of any new lesion. Individual lesions must be grater than 2 cm diameter by radiographic evaluation or grater than 1 cm diameter by physical examination.

Participants were treated with a saturated solution of potassium iodide (KI), Lugol's solution, or KI tablets orally starting at least 24 hours prior to the first infusion of Iodine I-131 Tositumomab (TST) and continuing for 14 days following the last infusion of Iodine I-131 TST. Participants received treatment in two phases. The dosimetric dose was administered in Phase 1 as 450 milligrams (mg) of TST infused over 1 hour, followed by 35 mg of antibody containing 5 millicurie (mCi) of Iodine I-131 TST infused over 20 minutes (min), followed by a 10-min normal saline flush. The therapeutic dose, administered 7-14 days after the dosimetric dose, was administered as 450 mg of TST infused over 1 hour, followed by 35 mg of antibody radiolabeled with enough Iodine I-131 TST to deliver 75 centigrey (cGy) infused over 20 min, followed by a 10-min normal saline flush.

Complete response (CR) is defined as the complete resolution of all disease-related radiological abnormalities and the disappearance of all signs and symptoms related to the disease. Response is defined as the best response achieved at any evaluation. A confirmed response is defined as a response that was confirmed by two separate response evaluations occurring at least 4 weeks apart.

ITT-Exposed Population: all participants who received at least one dose of study drug

Participants were treated with a saturated solution of potassium iodide (KI), Lugol's solution, or KI tablets orally starting at least 24 hours prior to the first infusion of Iodine I-131 Tositumomab (TST) and continuing for 14 days following the last infusion of Iodine I-131 TST. Participants received treatment in two phases. The dosimetric dose was administered in Phase 1 as 450 milligrams (mg) of TST infused over 1 hour, followed by 35 mg of antibody containing 5 millicurie (mCi) of Iodine I-131 TST infused over 20 minutes (min), followed by a 10-min normal saline flush. The therapeutic dose, administered 7-14 days after the dosimetric dose, was administered as 450 mg of TST infused over 1 hour, followed by 35 mg of antibody radiolabeled with enough Iodine I-131 TST to deliver 75 centigrey (cGy) infused over 20 min, followed by a 10-min normal saline flush.