1. Lysosomal proteinase activity was assayed in human cadaver kidney, urine, granules from polymorphonuclear leucocytes of normal persons, and urine samples from 154 patients with renal disease.
2. Granules from polymorphonuclear leucocytes showed proteinase activity at acid and neutral pH, whereas cadaver kidney showed proteinase activity at acid pH only.
3. The urine from 13 patients with glomerulonephritis showed proteinase activity at both acid and neutral pH as well as increased amounts of antigenic glomerular basement membrane fragments. The properties of the urinary proteinases suggested that they had originated in polymorphonuclear leucocytes.
4. Only the urine samples containing these proteinases were capable of degrading isolated human glomerular basement membrane in vitro.
5. Clinical recovery, where it occurred, was accompanied by the disappearance of urinary proteinase activity, and reduction in glomerular basement membrane antigen excretion.