Synthetic polycations bind to glomerular polyanions (GPA) and increase permeability to macromolecules and immune complexes. Platelet factor 4 and other platelet cationic proteins also bind to GPA and may play a role in immune complex deposition. Here we examine the potential of locally released cationic proteins to bind to GPA after immune complex-induced platelet activation within the renal microvasculature. Rabbits were immunized against bovine serum albumin (BSA), and BSA (2 or 4 mg/ml in buffered saline) was infused into the left renal artery to deliver 8 or 16 mg of BSA over 20 minutes. Thirty minutes later, kidneys were removed and tissue processed for light, immunofluorescence, and electron microscopy for the assessment of glomerular alterations and the localization of immune complexes (IgG and BSA), platelet factor 4, and platelet cationic proteins. GPA was measured by quantitative ultrastructural assessment of polyethyleneimine binding sites. Glomerular capillaries contained large intraluminal immune complexes and platelet aggregates. Also, numerous deposits were observed within subendothelial and subepithelial aspects of the glomerular basement membrane (GBM). Immunofluorescence and immunocytochemistry revealed prominent localization of platelet factor 4, platelet cationic proteins, IgG and BSA within peripheral capillary walls of glomeruli concomitant with a reduction in GPA. Glomeruli of controls or contralateral kidneys did not show GBM localization of immune complexes or platelet proteins. Thus, nascent formation of immune complexes in capillaries was associated with platelet activation and deposition of endogenous cationic proteins in the GBM. This mechanism may be involved in neutralization of GPA and mediation of increased permeability, which leads to GBM deposition of immune complexes.