This is a single-arm open-label study to verify dosimetry methods and determine the response rate, time to treatment failure, safety, efficacy, and dosimetry of radioimmunotherapy with Anti-B1 radiolabeled with 131 Iodine in patients who have chemotherapy-refractory low-grade non-Hodgkin's B-cell lymphoma or low-grade lymphoma which has undergone histologic transformation from low-grade to intermediate- or high-grade (transformed lymphoma).