An immunoglobulin M with kappa light chains (IgMK) pyroglobulin from a patient with hyperviscosity syndrome, erythrocytosis and coagulation defects has been studied for its immunochemical properties. At physiologic temperatures the purified macropyroglobulin showed a striking tendency to aggregate in the pentamer as well as in the monomer form. This property was also observed in its H chains. Aggregate formation of the pentamers may have contributed to the blood viscosity and coagulation defects. Formation of pyrogel at 56 °C was observed with pentamers as well as monomers, but not with separated H or L chains. Amino acid analysis showed quantitative abnormalities of aspartic acid, glycine, cystine and leucine within the H chains. Solubility of the pyrogel in sodium dodecyl sulfate, the pyroglobulin's tendency to aggregate and the cystine deficit of H chains implicate conformational changes leading to hydrophobic bonding at 56 °C in the formation of pyrogel.