SUMMARYKidneys were transplanted from Lewis and Brown Norway F1hybrid rats to Lewis recipients, and glomerular alterations were examined serially by light, electron, and immunofluorescence microscopy. Glomerular hypercellularity develops 3-5 days after allografting. Granular deposits of immunoglobulin G (IgG) and β1e-globulin (C3) occur at this time on glomerular capillary walls and mesangial areas, and subepithelial “humps” of basement membrane-like material are located on glomerular capillaries. The glomerular IgG and C3 deposits are short lived. Numerous polymorphonuclear leukocytes appear within glomeruli, strip away endothelium from basement membranes by extending pseudopods between endothelial cytoplasm and basement membrane, and accumulate IgG and C3 within their cytoplasm. On day 7, when renal function ceases, glomeruli are hypocellular and glomerular capillaries are occluded by platelets and fibrin. The results suggest (1) development of an immune complex glomerulitis during acute allograft rejection and (2) fibrin deposition during allograft rejection.