Cryoglobulin is an abnormal serum protein which precipitates or gels on cooling. Small quantities of cryoglobulin have been found in the sera of patients with many disparate diseases,1including subacute bacterial endocarditis, hepatic cirrhosis, blood dyscrasias, and the collagen diseases, but clinically significant amounts of cryoglobulin occur in only two conditions: multiple myeloma and "essential" cryoglobulinemia. In the test tube the serum from a cryoglobulinemic patient will show a flocculent white precipitate or opaque gelification when the temperature is reduced to 5 C. The rapidity of the solidification is proportional to the concentration of cryoglobulin and is graded from 1+ to 3+ on this basis. At cryoglobulin concentrations of 25 mg. per 100 cc. and over (3+), precipitation occurs immediately on chilling and may occur before the serum is cooled to below room temperature.1Repeated cooling and warming of cryoglobulinemic serum results in the eventual loss of the