Experience with a total of 49 plasma exchanges in 14 patients is reported. All exchanges were performed with the Aminco cellseparator in an attempt to remove documented or suspected antibodies completely. Indications were ABO-barrier before bone marrow-transplantation (6), acute rejection following renal transplantation (3), autoimmune diseases (3), Moschowitz disease, antithrombocytic antibodies and hepatic coma (1 each). The technique was modified to make it possible routinely to exchange 121 of plasma in 4 hours with no major untoward side effects even in anuric or thrombopenic patients. Complete removal of circulating antibodies was possible and often followed by impressive clinical benefit.