The M-component in the serum of a patient with multiple myeloma and hyperviscosity syndrome was found to be an immunoglobulin of the IgG-1(K) class which crystallized spontaneously from the serum. After purification, the protein crystallized readily under a variety of conditions. X-ray diffraction studies indicated a rhombohedral lattice with cell dimensions: aR = 170 ± 5 Å, αE = 109 ± 2°. Assuming a molecular weight of 1.5 × 105, the crystals have been calculated to contain 66 ± 3% solvent, and six molecules per unit cell.Screening of the protein for antibody-like activity against a wide range of antigens gave positive results in several instances, in particular when tested against gum Karaya. This reaction was inhibited by D-glucuronic acid.