Circulating immune complexes (CIC) have been studied in one or several serum samples obtained from 249 patients with cancer. 42.4% of patients with evolutive tumors and 26.9% of patients in complete remission have elevated levels of CIC. It appears that, in approximately one case out of 3, the presence of CIC is not necessarily in relationship with the tumor, but could be the consequence of other clinical factors. In some instances, repeated plasmapheresis results in decreased CIC levels. The specificity of the antibodies present in CIC is not yet known.