The relationship of Bence Jones protein (mol wt = 45,000) to a β2A-myeloma protein (mol wt = 160,000) formed by the same mouse plasma cell tumor (MPC-2) was investigated. The β2A-myeloma protein was split by treatment with papain and cysteine into fragments (S20,w = 3.7S), similar in size to the Bence Jones protein (S20,w = 3.6S). Two types of fragments with distinct antigenic groupings designated S and F, were present in the MPC-2 myeloma protein digest. These were partially separated by DEAE-cellulose chromatography. The Bence Jones protein was found to share antigenic determinants with S fragments from the MPC-2 β2A-myeloma protein and with S fragments from γ-globulins. Physicochemical observations indicated, however, that the Bence Jones protein was not identical to the globulin fragments produced by treatment with papain and cysteine. Comparison of the S and F fragments from β2A- and γ-globulins revealed that the antigenic features shared by the various globulins derived from plasma cells (γ- and β2A-myeloma proteins, the range of normal γ-globulins) are largely properties of the S fragments, whereas the distinctive antigenic differences between the γ- and β2A-myeloma proteins were properties which appeared in the F fragments of the molecules
