We developed a "tissue negative staining method" to observe the molecular-level ultrastructure in situ in any portion of the ultrathin sections routinely prepared for electron microscopy. This method was used in electron microscopy of the glomerular basement membranes (GBM). The GBM in patients with nephrotic syndrome was discovered to possess a tunnel structure, designated as "nephrotic tunnel", with lumen large enough to allow free passage of protein molecules. This tunnel seemed to be involved in the etiology of nephrotic syndrome. This new method appears to be applicable to a variety of purposes in biological studies.