Patients will be randomly assigned to one of two treatment arms. In Arm A, patients will receive 375 mg/m2 of rituximab (US, Canada – Rituxan®; EU - Mabthera®), given as an IV infusion once weekly for 4 weeks. In Arm B, patients will receive Iodine I 131 Tositumomab therapy. For Arm B, patients undergo a two-phase treatment. In the first phase, termed the “dosimetric dose,” patients will receive an infusion of unlabeled Tositumomab (450 mg) immediately followed by an infusion of Tositumomab (35 mg) that has been labeled with 5 mCi (0.18 GBq) of iodine 131. 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. Using the dosimetric data from these three imaging timepoints, the patient’s weight, and platelet count, a patient-specific administered activity of iodine I 131 Tositumomab (expressed in mCi or GBq) will be calculated to deliver the desired total body dose of radiation (65 or 75 cGy). In the second phase, termed the “therapeutic dose,” patients in Arm B will receive an infusion of unlabeled Tositumomab (450 mg) immediately followed by an infusion of the patient-specific administered activity of Iodine 131–conjugated Tositumomab (35 mg).