Over 100 patients have received cyclic treatment with polyclonal 131I labeled anti-ferritin and anti-carcinoembryonic antigen (CEA) antibodies from different animal species (rabbit, pig, cynomolgous monkey, bovine, and baboon). Because survival was prolonged from original cyclic treatment, retreatment with original antibodies (recycling) became a necessary consideration. An assay using autoradiography of Ouchterlony gels, with diffusion of patients' sera against the varied radiolabeled antibodies, was developed to detect anti-antibody precipitin bands. Anti-antibody could be detected with a sensitivity to the 60 ng level. Sera from 35 patients given from 1 to 7 separate cycles (2 injections/week, total antibody 6 mg/cycle) of radiolabeled foreign antibody were studied for the production of anti-antibodies. Anti-antibodies were detected in 11 of 22 primary hepatoma patients studied, 3 of 4 intrahepatic biliary cancer patients, and 0 of 9 Hodgkin's disease patients. In all but two of the patients, the anti-antibodies produced were specific for the species used in the treatment of the patient. Eight patients were reinjected (recycled) with previously used antibodies and the presence or absence of precipitin bands correlated with the ability of these antibodies to deposit in the tumor or to be rapidly degraded. The importance of this assay is its simplicity, sensitivity, and the rapid detection of anti-antibody activity for patients requiring treatment with radiolabeled antibodies.