Circulating immune complexes (CIC) were quantitated by a Raji cell radioimmunoassay in sera from Brown Norway rats bearing progressing or regressing methylcholanthrene-induced sarcomas. Quantitative profiles of CIC over time were related to tumor dose, tumor mass, and the regressive or progressive course of tumor growth. Animals bearing progressing tumors demonstrated an initial peak of CIC levels by 7 weeks but thereafter displayed a persistent decline in quantities of CIC despite continued tumor growth. Animals bearing regressing tumors were found to have a more directly proportional relationship between the tumor mass and the levels of CIC, but the appearance and disappearance of CIC lagged slightly behind the appearance and disappearance of the tumor. These data are almost identical to data previously published detailing quantities of CIC in BN and Lewis rats bearing Moloney sarcomas and are similar to fluctuations in CIC observed in animals and humans hosting a number of disparate neoplasms.