The immunochemical characteristics and the physicochemical properties of a cryoglobulin, as well as the hemodynamic and metabolic adaptations to the circulating protein, were studied in a patient with Raynaud's disease. The cryoglobulin was an IgG with lambda-type L and H chains of the δ1 subgroup with one Gm factor (No. 4) on the H chain. It formed gel in bulk from the serum or plasma at 37 °C or below and caused an increase in blood viscosity and in arteriovenous differences for oxygen, carbon dioxide and glucose, and at 15 °C increased production of lactic acid and preferential utilization of oleic acid by muscle tissue. Temperature changes alone probably are insufficient to produce gel formation in vivo; however, if gel formation did occur, it would be determined predominately by the unique hydrodynamic properties of the cryoglobulin in conjunction with local changes in blood rheology.