This is a single-arm, multicenter, expanded access study of Iodine-131 Anti-B1 Antibody for patients with relapsed or refractory low-grade or transformed low-grand non-Hodgkin's B-cell lymphoma (NHL). The primary objective of this study is to make Iodine-131 Anti-B1 Antibody more broadly available to patients. Secondary endpoints of the study will be to obtain additional information on the efficacy and safety of Iodine 131 Antibody. There will be two dosing phases in this study and then up to two years of follow-up. In the first phase, termed the "dosimetric dose", patients will receive an infusion of unlabeled Antibody (450 mg) over 60 minutes immediately followed by a 30 minute infusion (including a 10-minute flush) of Antibody (35 mg) which has been trace-labeled with 5 mCi of Iodine-131. Whole body counts using a gamma camera will be obtained 3 times between Days 0 and 7 following the dosimetric dose. The whole body counts will be used to determine a patient-specific mCi dose of Iodine-131 calculated to deliver the desired total body dose of radiation (either 65 or 75 cGy). The second phase, termed the "therapeutic dose", is administered 7-14 days after the dosimetric dose. Patients will receive a 60-minute infusion of unlabeled AntiB1 Antibody (450 mg) immediately followed by a 30-minute infusion (including a 10 minute flush) of Antibody (35 mg) labeled with the patient-specific dose of Iodine-131 (median dose in previous studies was approximately 85 mCi). Patients who are obese will be dosed based upon 137% of their calculated lean body mass. Patients will be treated with either saturated solution potassium iodide (SSKI), Lugol's solution, or potassium iodide tablets starting at least 24 hours prior to the first infusion of the Iodine-131 Antibody (i.e., dosimetric dose) and continuing for 14 days following the last infusion of Iodine-131 Antibody (i.e., therapeutic dose). Hematology, blood chemistry and adverse experiences will be followed for 13 weeks. Response evaluations will be performed at weeks 13 and 25 and then every 6 months until disease progression, patient death, or for two years, which ever occurs first. Thyroid function will also be followed.