OF the three layers of the glomerular capillary wall—endothelial cells, basement membrane and epithelial cells—the basement membrane appears to be the only complete barrier between the plasma and the glomerular filtrate1,2. Support for the view that the basement membrane is the most important filtering mechanism was provided by the studies on ferritin transfer across the glomerular capillary wall2–5. Graham and Karnovsky6 have concluded that the epithelial slits are the primary filtration barrier and suggested that glomerular basement membrane acts only as a “coarse” filter. It is thus still not established whether the basement membrane acts as a filter.