We have studied the phagocytic function of polymorphonuclear leukocytes obtained from 16 patients with essential mixed cryoglobulinaemia and 16 normal subjects, utilizing three different experimental models. The in vitro phagocytosis of Candida albicans by polymorphonuclear leukocytes obtained from the patients in the presence of autologous serum was subnormal as compared with controls (p less than 0.002). The neutrophil defect appeared to be cellular since: a) no effect of the patient's serum on the phagocytic capacity of normal neutrophils was demonstrable; b) the defect was not reversed when normal AB serum was used as opsonin source. The polymorphonuclear leukocytes from the same patients ingested IgG-coated sheep red blood cells to a significant higher degree than normals (p less than 0.002) when tested in patient's serum free-medium. Finally, no differences between patients and controls were observed on the basis of latex uptake. A possible explanation for these contradictory results is discussed.