An ultrafiltration system for the purification of toxic substances is demonstrated. Using this system, it is possible to eliminate substances with a molecular weight up to 5,000 in the same amount as smaller molecules. The clearances for smaller molecules are lower and for larger molecules higher compared to haemodialysis and they depend mainly on the ultrafiltrate flow. Amino acid loss did not exceed the amount which was eliminated by plate haemodialysers. Side effects, such as excessive haemolysis, platelet or fibrinogen depletion, were not to be found.