During an 11-month period, 270 cryoglobulins were identified in 735 sera from patients versus 34 in 82 sera from apparently healthy blood donors. Western-blot analysis of the cryoglobulins was performed and cryoglobulin protein content was determined using a colorimetric assay. Comparison of findings in the two groups suggested that a substantial proportion of cryoglobulins identified in the sera of patients are quantitatively and qualitatively indistinguishable from cryoglobulins identified in healthy controls.