A Bence-Jones protein which crystallized spontaneously from the urine of a patient (R.) at the Mayo Clinic in 1920 furnished material unusually well suited to the investigation of some of the problems associated with Bence-Jones proteinuria. With this specimen of the protein, which was purified by recrystallization, immunological studies were undertaken to discover, if possible, (1) a difference between various specimens of Bence-Jones protein, and (2) a difference between Bence-Jones protein and the proteins of human blood serum.In the past, the few immunological studies on Bence-Jones protein have been directed solely toward the differentiation of human serum proteins from Bence-Jones protein. With the exception of Massini (1911), who was able to show by complement fixation tests specific distinctions between human serum and Bence-Jones protein in different zones of dilution, these investigations have apparently indicated that Bence-Jones protein and human serum proteins are immunologically indistinguis...