1. Human polymorphonuclear leucocyte elastase and cathepsin G were incubated with preparations of isolated human glomerular base ment membrane at neutral pH and 37 C. 2. The ability of these enzymes to degrade glomerular basement membrane was followed by the release of hydroxyproline. Both proteinases released considerable amounts of hydroxyproline. 3. By using Sephadex G-100 it was shown that the solubilized basement membrane fragments appeared as a single peak and had a molecular weight of over 100 000. These proteins after reduc tion were analysed by sodium dodecyl sulphate-gel electrophoresis to examine their subunit pattern and determine their molecular size. 4. The released basement membrane proteins gave at least four precipitin lines with a rabbit anti-(glomerular basement membrane) antiserum. 5. These results support the concept that poly morphonuclear leucocyte neutral proteinases play an important role in the pathogenesis of glomerulo nephritis.