In a previous study we examined the role of antigenic electrical charge as a factor influencing glomerular immune complex (IC) localization in the rabbit. From that study patterned after chronic serum sickness nephritis it was demonstrated that the administration of charge-modified cationic bovine serum albumin (BSA) of isoelectric point (pI) ≧ 9.5 invariably resulted in heavy subepithelial deposits, whereas native BSA (anionic, pI 4.5) produced principally mesangial deposits. In order to investigate the mechanism by which the immune deposits formed, passive serum sickness was induced in New Zealand white rabbits by unilaterally perfusing kidneys of nonimmunized rabbits with five alternating cycles of saline, antigen and antibody, or immunized rabbits with saline and antigen alone. Localization of BSA and IgG along the glomerular basement membrane (GBM) occurred only after exposure to cationic BSA and antibody; non-immunized animals perfused with cationic BSA and antibody to cationic BSA uniformly developed generalized, diffuse nearly linear deposits of IgG and BSA along the GBM. Similar deposits, which became progressively more granular after exposure to circulating antibody, were seen after the perfusion of cationic BSA alone into animals actively immunized with cationic or native BSA. Ultrastructural examination showed effacement of foot processes and isolated, irregular subepithelial and subendothelial deposits. Control perfusion employing alternating cycles of native BSA and anti-native BSA antibody, cationic BSA and normal sheep IgG, or native BSA alone in animals actively immunized with native or cationic BSA failed to develop glomerular IgG deposits. All control kidneys were normal on ultrastructural examination. We conclude that epimembranous deposits induced by cationic BSA are formed locally in the GBM and that in the perfused rabbit kidney a cationic antigen is a requirement for in situ IC formation in the subepithelial space.