In vitro and in vivo dialysis with radio-iron added to the dialysate indicate that only a minimal amount of iron (approximately ¼%) is transferred from dialysate to patient across the cuprophane membrane. This amount corresponds to70µ g of iron per dialysis counting on the basis of 8µ g of iron per 100 ml in Los Angeles tap water utilized for dialysate preparation. Iron deposition in biopsy and autopsy material seems to be derived entirely from frequent blood administration.