Autologous immune complex nephritis (Heymann nephritis) was actively induced in rats by immunization with high (10 mg.) and low (1 mg.) doses of renal tubular epithelial antigen in complete Freund's adjuvant. The development of proteinuria and granular capillary wall deposition of IgG confirmed the previously well described membranous nephropathy which characterizes this experimental disease. Circulating immune complexes were demonstrated by both the fluid phase and solid phase C1q binding assays in both high and low dose experimental groups. The prevalence of such immune complexes was significantly greater in the experimental than in the control groups immunized with adjuvant alone or liver homogenate in adjuvant. The circulating immune complexes bound to C1q were 16 to 23 S in size and were proven to contain a renal tubular antigen. These data, in combination with the previous demonstration of renal tubular antigen and its antibody in kidneys from rats with autologous immune complex nephritis, are consistent with a circulating immune complex pathogenesis of this model of the actively induced autologous immune complex nephritis in rats.