Previous reports on the nephritogenic and immunological enhancing capacity of posttransplant circulating immune complexes (CICs) are conflicting and have been confined to the study of acute rejection episode (AR). This study was undertaken to assess the nephritogenicity of CICs in patients undergoing accelerated acute rejection (AAR) and chronic rejection (CR) episodes. We also assessed the possible role of CICs as mediators of immunological enhancement by assessing CIC levels in long-term-stable allograft recipients. To assess the nephritogenic role of CICs, 98 CIC determinations were performed on 49 serum samples from 41 pediatric renal transplant recipients using the Clq solid-phase assay (Clq-SPA) and the Raji cell radioimmunoassay (Raji-RIA). Serum samples from normal subjects served as controls. No recipient undergoing AAR had evidence of CICs by either assay. 5 of 21 (23.8%) recipients undergoing CR had positive CIC levels in the Raji-RIA, while 4 of 21 (19%) recipients had positive CICs in the Clq-SPA. There was no statistically significant correlation of CIC levels with long-term allograft function. In addition, there was no evidence supporting antithymocyte globulin as an immunogen in patients demonstrating posttransplant CICs. In summary, CICs do not appear to be an important mediator of AAR or CR episodes, and CICs were not routinely detected in patients with good long-term allograft function.