A new parallel plate membrane plasma exchange system (Centry TPE System) has been developed and clinically evaluated. A series of 27 consecutive plasma exchanges were performed on 14 patients with autoimmune diseases. Blood access was obtained by routine means identical to centrifugal procedures. The blood flow rate was 94 f 20 ml/min and plasma Filtration rate was 37 9 ml/min, with an average patient hematocrit of 36%. The sieving coefficients for a wide range of plasma solutes including immunoglobulins, immune complexes, and lipids, clustered about 1.00, indicating nonselective plasma filtration. The average reduction in patient plasma solutes of 60 to 70% was in close agrcement with the predicted value for unhindered plasma filtration. There was no appreciable loss of blood cells, as compared to a study with a centrifugal system showing a 56% reduction in circulating platelets. Complement activation measured by C3 conversion was undetectable in patient samples and either undetectable or minimal in plasma filtrate samples. The system was convenient to operate and problems were rare. N o untoward patient effect5 attributablc to thc membrane system were observed. These results show that the parallel plate membrane plasma exchange system described in this report is capable of delivering safe, efficient plasma exchange.