The localization of a platelet antigen or antigens kidneys from 106 patients with renal disease was evaluated with immunofluorescent microscopy by using a rabbit antibody to human platelets. The antiplatelet IgG fixed to the surface membrane of platelets did not react with erythrocytes, leukocytes, plasma, normal kidney, or a variety of normal tissue targets. Significant glomerular and vascular deposition of platelet antigen (or antigens) was observed along the endothelium or as vascular plugs in kidney tissue from patients with hemolytic uremic syndrome, membranoproliferative glomerulonephritis (types I and II), diabetic nephropathy, hypertensive renal disease, scleroderma, and other diseases. Dual-label immunofluorescent studies revealed that platelet antigen (or antigens) and fibrinogen/fibrin-related antigen (FRA) were usually, though not always, present in similar loci. Platelet antigens were not observed at sites of intense FRA deposition: in the mesangium in anaphylactoid purpura and in glomerular crescents in Goodpasture's syndrome. Platelet antigen was detected in the peritubular capillaries of most patients with diabetic nephropathy.