The detailed organization of normal glomerular capillaries as well as the changes in their organization which occur in association with many renal diseases are by now well known. However, the question of which structure in the glomerular capillary wall represents the primary filter serving to retain plasma proteins in the circulation has so far not been resolved. Two hypotheses have been proposed—one, that the basement membrane is the critical barrier, and the other that the basement membrane represents only a crude prefilter, with the critical barrier residing at the epithelial filtration slits or “slit-pores.”

Needless to say, it is very important to know if there is just one filtration barrier to macromolecules or two in series and especially to know if the critical, size-limiting barrier resides at the level of the basement membrane or the epithelial slits. The purpose of this editorial is to summarize the accumulated information on the glomerular filtration barrier and especially to critically examine the available evidence which would enable us to localize the filter among the glomerular structural elements.