Cathepsin D activity has been studied by a fluoremetric assay in the urine of acute post-streptococcal glomerulonephritic (APSGN) patients aged from 3 to 14 years and has been found elevated when compared with four groups of controls. This activity cannot be accounted for by erythrocytes and/or leukocytes in the urine of these patients since haematuric and pyuric controls did not exhibit an amount of enzyme activity greater than the normal control group. Cathepsin D activity can be attributed to lysosomal enzymes released from polymorphonuclear leukocytes which are in close contact with glomerular basement membrane. Complement C3 levels in serum and cathepsin D activity in urine of these patients showed no correlation.