 This multicenter study will determine the response rate, the complete response rate, duration of response, time to progression, time-to-treatment failure, safety, and survival following treatment with Iodine-131 Anti-B1 Antibody for the retreatment of patients with non-Hodgkins lymphoma who previously responded with a duration of response of at least 3 months to Iodine-131 Anti-B1 Antibody therapy. Patients 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. 