Reported previously in certain human carcinomas and rat and mouse experimental tumor systems, circulating immune complexes (IC) have now been detected in Syrian hamsters bearing tumors produced by injection with syngeneic TSV5Cl2 cells. IC were detected by the Raji cell radioimmune assay, adapted for use in hamster sera. A novel feature of this test is the use of a stable covalently linked hamster immunoglobulin G aggregate as the reaction standard. Stable over six months on storage at -70 degrees and showing no tendency to form precipitates on thawing or during test procedures, this preparation greatly facilitated quantitation of hamster IC by Raji cells. Seven of 19 sera of hamsters bearing SV40-induced tumors from 60 to 128 days had IC concentrations exceeding 40 microgram aggregated hamster immunoglobulin G equivalents per ml, as contrasted to IC levels of less than 25 microgram for 19 age-matched normal hamsters. There appeared to be no significant correlation between IC levels and tumor weight or duration of tumor within the hamster host. The results suggest a complex relationship between IC and a number of factors connected with tumor growth.