The circulating radioactivity and antibody immunoreactivity in patients with diverse cancers who had received 131I-labeled goat antibodies to carcinoembryonic antigen (CEA) were studied by Sephadex G-200 column chromatography and with solid-phase (SP) immunoadsorbents containing anti-goat immunoglobulin G(IgG), anti-human IgG, anti-CEA, or CEA. Upon gel filtration, more than 80% of the plasma radioactivity was distributed between native IgG and an excluded macromolecular radioactive fraction (Pool I). The native IgG and Pool I radioactive peaks were immunoreactive with the SP anti-goat IgG and SP CEA to the same extent as was the radioantibody prior to injection. The circulating CEA in patients with elevated titers only partially bound the injected radioantibody since less than 50% of the latter chromatographed as Pool I. Up to 50% of the Pool I radioantibody from this group of patients was bound to the SP anti-CEA, whereas it was minimally reactive with the SP anti-human IgG. The clearance of radioantibody was similar between groups having different amounts of Pool I radioantibody, and injection of CEA radioantibody was not accompanied by a decrease in circulating antigen. Patients with lower CEA titers had the majority of the plasma radioactivity chromatographing as Pool I radioantibody which showed elevated binding to the SP anti-human IgG but not the SP anti-CEA. Tumor localization by photoscanning was observed in seven of eight and nine of nine patients who had circulating CEA-radioantibody and anti-immunoglobulin-radioantibody complexes, respectively. Thus, these studies demonstrate that CEA, as well as human antibody reactive with goat IgG, can form immune complexes in patients given injections of CEA radiolocalizing antibody. However, these complexes do not appear to prevent tumor radioimmunodetection.