The first so-called "artificial kidney" used in this country was an ingenious device containing a collodion membrane as the filter and hirudin, the active anticoagulant principle of the leech, to prevent blood clotting. It was used in experiments to remove salicylate from the blood of dogs by extracorporeal circulation.1 Almost 50 years later the same basic technique is used to treat salicylate poisoning in the human.2 In spite of some radical and occasional bizarre changes in design, only one fundamental modification has been made in the apparatus proposed by Abel in 1913.1 This is the substitution of a cellophane membrane for the collodion membrane laboriously hand poured by Abel and his collaborators. The necessity for grinding leech heads to extract its anticoagulant substance has been obviated by the development of heparin. With this exception, little change has occurred in the fundamental operating principle of hemodialysis. As the