Tap water acting on copper plumbing contributes up to 1 ppm of copper to dialysis fluid in Australian hemodialysis units. Patients dialyzed against 0.4 ppm of copper show uptake of copper by both plasma and red blood cells during hemodialysis. In experimental dialysis of dogs against 0.4 and 1 ppm of copper, the initial uptake is mainly by the plasma, but copper concentration occurs in the red blood cells and in the liver. The possibility that, in patients on longterm hemodialysis, copper accumulation in the liver might eventually lead to hepatitis and hemolysis is discussed in relation to chronic copper poisoning of sheep and to Wilson's disease.