The serum of a patient with chronic lymphocytic leukemia, and amyloidosis, was found to contain an unusual protein with µ chain antigenic determinants. It was devoid of light chains and was present in the form of multiple disulfide linked polymers. The patient also had a kappa Bence Jones protein in the serum and urine. The possibility is raised that the heavy chain lacks a portion of the chain necessary for coupling with light chains.