At the time of the initial diagnosis the serum of a myeloma patient showed a typical monoclonal protein spike of the electrophoretic mobility of γ2-globulins. Large amounts of light chains were excreted in the urine. They traveled electrophoretically with the migration of β2globulins. As the patient developed progressive renal failure, a "new" second peak appeared in the serum. The electrophoretic migration of this new peak was identical to that of the urinary Bence Jones proteins. Double immune diffusion, immunoelectrophoresis, ultracentrifugation, and analysis of amino acid composition proved the identity of the new serum peak with the urinary Bence Jones proteins.