 This multi-center, single-arm study of epratuzumab is in patients with Waldenströms Macroglobulinemia WM who failed chemotherapy. After baseline evaluations, patients receive epratuzumab infused over approximately 30-60 minutes at 360 mgm2, administered once weekly for 4 consecutive weeks days 1, 8, 15, 22. Post-treatment evaluations occur on the day of the last infusion, then at 6 and 12 weeks. Patients without progression of disease continue long-term follow up until disease progression or for at least 5 years, with evaluations every 3 months for 2 years, then semi-annually. Otherwise, follow-up is only required until resolution of any treatment related abnormalities. 