Urinary excretion of acid glycosaminoglycans (AGAG) was estimated in 12 patients with Alport's syndrome (AS), 10 patients with idiopathic nephrotic syndrome and 10 healthy children. Comparing children with AS and healthy subjects or patients with idiopathic nephrotic syndrome, significant differences could be detected. Glomerular basement membrane antigen excreted into urine showed in 12 out of 12 patients with AS altered immunoelectrophoretic mobility into the beta-zone thus correlating with increased AGAG excretion. 10 patients with idiopathic nephrotic syndrome and 10 healthy children, showing normal AGAG excretion rates, presented normal glomerular basement membrane antigen with alpha-2-mobility.