Sera from 517 patients with various types of malignancies were assayed for immune complexes (ICs) by the Raji cell radioimmune assay. The incidence of immune complexes in these patients ranged from 16 to 52% as compared to 19% in normal controls. Increases in tumor mass and metastatic disease were associated with high levels of circulating ICs. Immunization of melanoma patients with BCG and tumor-cell vaccine produced an increase in levels of ICs. Cancer sera contained complexes of intermediate size. Tumor antigens and IgG, presumably in the form of complexes, were identified by immunofluorescence and radiolabeled antibody techniques on the surface of Raji cells incubated in ICs-containing cancer sera.