Comparison of rituximab versus Iodine I 131 Tositumomab therapeutic regimen in subjects with follicular non Hodgkins B cell lymphoma. 506 subjects will be enrolled at 30 to 40 sites in the US, Canada, and Europe. Subjects will be randomly assigned to one of two treatment arms. In Arm A, subjects will receive 375 milligrams/meter2 (mg/m2 )of rituximab, given as an intravenous (IV) infusion once weekly for 4 weeks. In Arm B, subjects will undergo a two-phase treatment. In the first phase, termed the "dosimetric dose," subjects will receive an infusion of unlabeled Tositumomab (450 mg) immediately followed by an infusion of 5 millicuries (mCi) (0.18 gigabecquerel [GBq]) of Iodine 131 Tositumomab (35 mg). Whole body gamma camera scans will be obtained three times (Day 0; Day 2, 3, or 4; and Day 6 or 7) following the dosimetric dose. The information derived from the scans will enable a patient specific dose to be calculated to deliver the desired total body dose of radiation (65 or 75 centigray [cGy]). In the second phase, termed the "therapeutic dose," subjects in Arm B will receive an infusion of unlabeled Tositumomab (450 mg) immediately followed by an infusion of the subject specific activity of Iodine 131-conjugated Tositumomab (35 mg). Thyroid blockade will be implemented 24 hours prior to the dosimetric dose and continued for 14 days following the therapeutic dose. Subjects on study will be followed for response and safety at Week 7, Week 13, and every three months for the first and second year, every six months for the third year, and then annually for the forth and fifth years; and then for vital status, additional therapy, and long term safety events through year ten. Follow Up after subsequent NHL therapy will be carried out to assess tolerance of next anti-lymphoma therapy, development of myelodysplasia (MDS)/acute myelogenous leukemia (AML), HAMA or hypothyroidism, unexpected safety issues, and death.