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The unique myeloma protein from S. J., a patient with multiple myeloma, was isolated and characterized. It resembled other myeloma proteins in many respects. The S. J. myeloma protein migrated in a distinct peak in the slow β-globulin region on zone electrophoresis, appeared as a single band on starch gel electrophoresis, and sedimented at 7.04S in the ultracentrifuge. Papain and cysteine treatment produced Fc (fast) and Fab (slow) fragments. Reduction and alkylation of the myeloma protein produced heavy and light chains in a ratio of approximately 3:1. The S. J. myeloma protein had type L (type II) light chains. These were antigenically similar to the Bence Jones protein also found in this patient. The S. J. myeloma protein was unique in the properties of its heavy chains. The myeloma protein (and its heavy chains and Fc pieces) did not contain antigenic determinants specific for IgG, IgA, or IgM. The myeloma protein (and its heavy chains), however, did contain antigenic determinants which are characteristic of a new class of immunoglobulin. The S. J. myeloma protein was unusual also in its effect on the metabolism of normal IgG and in the electrophoretic mobility of the Fc fragment produced by papain digestion. No evidence was obtained to indicate that the entire heavy polypeptide of the S. J. protein was a grossly abnormal product of malignant cell metabolism. The unique properties of the S. J. myeloma protein (and its heavy chains) are believed to represent, in large measure, properties to be found in a small part of the normal immunoglobulin population.