This is a phase II, open-label, multicenter study of the efficacy and safety sequential administration of cycolophosphamide, vacristine, and prednisone (CVP) x6 cycles followed by tositumomab and iodine I 131 tositumomab for previously untreated subjects with low-grade Non-Hodgkin's Lymphoma (NHL). CVP will be repeated every 21 days for a total of six cycles. tositumomab and iodine I 131 tositumomab will begin within 56 days following the first day of the sixth cycle of CVP. Subjects will undergo two dosing phase for the tositumomab and iodine I 131 tositumomab therapy. In the first phase, "dosimetric dose", patients will receive an infusion of unlabeled Anti-B1 Antibody (450mg) over 60 minutes followed by a 30 minute infusion (including a 10-minutes flush) of Anti-B1 (35mg) containing 5mCi of Iodine-131. Whole body gamma camera scans will be obtained on Day 0; Day 2, 3, or 4 and Day 6 or 7 following the dosimetric dose. Using the dosimetric data from three time points, a patient-specific dose of Iodine-131 will be calculated to deliver the desired total body dose of radiotherapy. In the second phase, termed the "therapeutic dose", patients will receive 60-minute infusion of unlabeled Anti-B1 Antibody (450 mg) followed by a 30-minute infusion (including a 10-minute flush) of 35 mg Anti-B1 Antibody labeled with the subjects -specific dose of Iodine-131 calculate to deliver a 75cGy total body radiation dose. Subjects who have platelet counts of 100,000-149,999 cells/mm3 will receive 65 cGy; obese patients will be dosed base upon 137% of their lean body mass. Subjects will be treated with saturated solution potassium iodide (SSKI), Lugol's solution, or potassium iodide tablets starting at least 24 hours prior to the first infusion of the tositumomab and iodine I 131 tositumomab (i.e., the dosimetric dose) and continuing for 14 days following the least infusion of tositumomab and iodine I 131 tositumomab (i.e., the therapeutic dose).