Bence Jones proteinuria, a common clinical manifestation of multiple myeloma, can also be seen in patients with other B-cell-derived neoplasms. Measurement of pretreatment levels is a useful adjunct in the diagnosis and staging of multiple myeloma, whereas serial levels reflect response to therapy. Serum concentrations of beta2-microglobulin, a small-molecular-weight protein associated with the major histocompatibility complex class I antigens, are often elevated in hematopoietic neoplasms and are also commonly measured at baseline, before treatment, and serially throughout therapy in patients with multiple myeloma and other lymphoproliferative disorders as a marker of tumor burden. Urinary concentrations, however, are considered an indicator of renal tubular function. High urinary levels are found in tubular proteinuria, a frequent sequela of long-standing multiple myeloma. A case is described in which a high urinary concentration of beta2-microglobulin interfered with Bence Jones protein quantification by electrophoresis studies.