Rat kidneys were treated with chromatographically purified neuraminidase (Clostridium perfringens) in order to determine the effect of removal of sialic acid on glomerular structures. Kidneys were perfused with enzyme for 30-60 min at 37 C, after which they were fixed by perfusion and stained with various cationic probe molecules-colloidal iron, ruthenium red, Alcian blue, or cationized ferritin. Perfusion with neuraminidase resulted in the progressive detachment of the endothelium and epithelium from the glomerular basement membrane (GBM). In colloidal iron preparations there was a complete absence of staining from both the endothelium and epithelium indicating a total removal of sialic acid from the cell surfaces. Free sialic acid was also detected in the bladder fluid. The binding of cationic probes to the anionic sites in the GBM which have been shown to consist of glycosaminoglycans (heparan sulfate) was unaffected by neuraminidase treatment; however, the lamina densa of the GBM appeared thinner and somewhat more pale-staining in places than in control specimens. Detachment appeared to be due to the disruption of a network of filaments which normally extend from the GBM to the adjoining cell surfaces. Other agents tested which were also found to cause cell detachment when perfused into glomeruli were denaturing agents (urea, guanidine) and proteolytic enzymes (trypsin). Perfusion of ethylene-diaminetetraacetic acid (EDTA) or strong acidic or basic buffers had no effect on attachment. The results indicate that the presence of sialic acid is necessary for maintenance of the attachment of the overlying cells to the GBM, whereas divalent cations and ionic interactions do not appear to play a major role. The findings are discussed in relation to known sialic acid-containing macromolecules (fibronectin? laminin?) which may be involved in attachment, and in relation to the similarity of the effects produced by neuraminidase perfusion to findings in aminonucleoside nephrosis and other glomerular diseases.