Of 260 paraproteinemic sera 8 contained pyroglobulins: 1 IgG-myeloma, 1 IgA, 6 Waldenström’s disease, 3 with and 3 without cryoglobulinemia. The clinical and serological findings of these patients are described. Electrophoresis and separation of immunoglobulins suggest that the M-component is the pyroglobulin. In mixed cryoglobulinemias IgM- IgG, only the IgM is the pyroprecipitable component. The thermolability of pyroglobulins is due to a complex mechanism resulting from intramolecular or intermolecular reactions; itis not necessarily linked only to the polymerization at the level of disulfide bonds