A molecular filtration procedure for preparing large quantities of human erythrocyte ghost membranes is presented. Hemolysate ghost membranes are rapidly cycled in the retentate channel of the filtration apparatus, while hemoglobin is removed as it passes through Pellicon filters into the filtrate channel. Several-liter quantities of washed packed erythrocytes can be processed in a few hours with this system, and the filtration procedure does not appear to alter intact erythrocyte or ghost membranes. Intact erythrocytes in isotonic solution can be circulated through the retentate channel for 16 h with only 3% hemolysis and with preservation of their original morphology in scanning electron microscopy. Ghost membranes isolated by the procedure are virtually identical in morphology, polypeptide composition and acetylcholinesterase content to membranes isolated by conventional centrifugation techniques.