Circulating Immune Complexes (CIC) have been described in malignant diseases. To elucidate their potential clinical significance, CIC of seventy-eight patients with cancer and of seventy-one normal individuals were studied. Total protein and immunoglobulin composition of CIC were quantitatively measured. Patients sera had consistently higher titers of CIC than normal individuals. Sodium Dodecyl Sulfate/Polyacrylamide Gel Electrophoresis (SDS/PAGE) showed subtle differences in the CIC protein composition between patients and normal individuals.