Sprague-Dawley rats, 6 with aminonucleoside nephrosis and 6 controls, were intravenously injected with human liver ferritin isolated from post mortem liver, and their 24-h urine samples were examined for human ferritin by immunoradiometric assay. In rats with aminonucleoside nephrosis, the amount of excreted ferritin in urine was forty times greater than in control rats. Much more monomeric ferritin was excreted than that of polymeric ferritin. We are the first to have utilized human liver ferritin as a tracer to measure a minor amount of ferritin by a commercially available kit. Our present study seems to indicate a critical role for glomerular basement membrane as a size barrier.