Heymann nephritis of the rat has several similarities to human idiopathic membranous glomerulo-nephropathy, and therefore serves as an animal model for study of the human disease. The disease process in the rat is initiated by autoantibodies forming in situ immune deposits on the surface of the glomerular epithelial cell in the lamina rara externa region of the glomerular basement membrane. It is now understood that multispecific antibodies (like anti-gp600) form the nephritogenic deposits which cause proteinuria, while MoAb (like anti-gp330, anti-90 kD and anti-gp70) form only transient deposits which do not give rise to proteinuria. The reasons for differences between pathologic effects of mono- and multispecific antibodies are not known. Following characterization of putative antigens of Heymann nephritis by immunofluorescence, immunogold and immunoblot techniques, we have investigated the metabolic handling of a multispecific (anti-600), monospecific antibody (anti-gp70) and a MoAb (anti-gp330) by the cultured glomerular epithelial cell to gain an insight into the mechanism of nephritogenicity of multispecific antibody. Anti-gp600 reacted to multiple antigens in the 330-kD and 70-kD regions; anti-gp70 reacted to only the 70-kD region. Ultrastructurally, all three types of antigens were present on the plasma membrane, concentrated in the microvillar region. The cells were incubated with the antibodies, and clearance of the antibodies from the cells was evaluated. Following binding, all three antibodies were internalized by the cells. However, it was found that monoclonal anti-gp330 MoAb was cleared most rapidly from the cell (t1/2 5 min), followed by anti-gp70 (t1/2 30 min). Anti-gp600 was cleared at four times slower rate than anti-gp70 (t1/2 2h) by the cell. While anti-gp330 MoAb and monospecific anti-gp70 antibody were expelled from the cell in 25-32% digested form, the multispecific anti-gp600 was digested to the extent of only 5-8%. No immune complexes were detected in the medium with any of the three antibodies. The shed label was in intact IgG form. It is concluded that multispecific antibodies of Heymann nephritis are nephritogenic because of their slower clearance and digestion and therefore, higher accretion rate on the surface of the glomerular epithelial cell. The differential handling of the antibodies by the glomerular epithelial cell offers an explanation for the in vivo differences in the nephritogenicity of these antibodies observed earlier.