The results from Phase 1/2 (RIT-I-000) and Phase 2 (RIT-II-001) studies of Tositumomab and Iodine I 131 Tositumomab (TST/I-131 TST) demonstrated that TST/ I-131 TST produced a high response rate in patients with chemotherapy-relapsed/refractory, low-grade or transformed low-grade Non-Hodgkin's Lymphoma (NHL). On the basis of these results this study was designed to compare the efficacy of TST/ I-131 TST to the last qualifying chemotherapy regimen in patients with chemotherapy-refractory, low-grade or transformed low-grade NHL.