Subjects were randomized to receive either tositumomab (Anti-B1 Antibody) and iodine I 131 tositumomab (Arm A) or unlabeled tositumomab (Arm B). Subjects randomized to Arm B were allowed to cross over and receive I 131 tositumomab once their disease had progressed as long as they still fulfilled the protocol entry criteria (except for exclusion criterion 12, prior monoclonal antibody therapy) and were human anti-murine antibody (HAMA) negative. Study endpoint assessments of response were conducted by a Masked Independent Randomized Radiographic and Oncologic Review (MIRROR) panel and the Study Investigators' assessments of safety and survival. Subjects who completed at least two years of follow-up in Protocol BEX104515 (formerly Corixa Protocol RIT-II-002) were enrolled in long term follow-up Protocol BEX104526 (formerly Corixa Protocol CCBX001-051), an administrative protocol, for continued radiographic response evaluations and safety evaluations every 6 months for years 3 through 5 post-treatment and annually for years 6 through 10 post-treatment. Subjects in BEX104526 were assessed for survival, disease status, subsequent therapy for NHL, and long-term safety, including the use of thyroid medication, development of hypothyroidism, human anti murine antibody (HAMA), myelodysplastic syndrome, acute myelogenous leukemia, and all other secondary malignancies. Additionally, subjects were followed for the development of any adverse event(s) deemed by the Principal Investigator as being possibly or probably related to a subject's previous treatment with Iodine I-131 tositumomab. Laboratory evaluations consisting of a thyroid stimulating hormone level and a complete blood cell count, with a differential and platelet count, were obtained annually through year 10 post-treatment.